I've got limited funds and even more limited access to a bookstore that carrys role-playing games, but I've got The Dragon and The Bear ordered! Considering I can't afford every suppliment (at least not right away) what else do you recommend?
The money in my pocket--it burns! It burns! Help me spend it!
If you have limited funds, concentrate on the supplements first... except maybe for the tribunal book where you are planning on playing.
I'd recommend the 3 Houses of Hermes books, then Mysteries, then Art and Academe, then the Realms of Power books (Divine and Infernal out, magic soon). Then whatever interests you from the rest.
You are in for an exceptional read. "The Dragon and the Bear" is one of the finest 4th Ed. sourcebooks, and my personal favourite of all the Tribunal books (the 5th Ed. "Guardians of the Forest" is in a close #2 position).
The biggest question is whether you're planning to play a blend of 4th/3rd Ed. or if you're making the jump into 5th Ed. I only recently launched into 5th Ed., and while there are canon breaks that leave me screaming, the overall quality of writing has been excellent.
You may want to consider shopping on eBay for some of the best discounts for the older books.
That said, I agree that if you're going with 5th Ed. then after the main book you'll want the three "Houses of Hermes" books, "The Mysteries", and possibly "Guardians of the Forest" (which is broadly compatible with "The Dragon and the Bear").
I'd recommend designing a campaign that suits what you can afford. You can play quite a good game without the House books (and I wrote part of all three, so this isn't knocking them at all.) and if you can only afford one, then True Lineages is the one. If you really have tight money problems, then, myself, I'd have perhaps Covenants (although you can make do without it really easily too) and RoP:Magic if you can wait for it. If not, I'd buy Infernal, just for the pregen enemies.
That is, the bells and whistles of the game are cool and fun, but a lot of us had some good sagas just with the basic book, and you can too. Buy whatever builds up the story you personally feel like telling.
I have to agree with that, it's an excellent supplement. As people have said, try ebay for 4th edition or earlier. But some stuff is also available to buy as pdf. I don't know if it's cheaper, but it's an option (personally, i prefer having the books, but ymmv).
Covenants, I think, is critical for anyone who wants to expand the customization of the game; HoH:True Lineages is also important, but almost exclusively for the Hermetic Breakthrough rules.
Beyond that, pick what you'll be doing most often. I'm working on a more mundane interaction-based saga, so I've been using RoP:Divine and City and Guild in a big way.
If you are familiar with the setting, or willing to make up setting, I agree with this. More generally I think the bit on the law is handy too.
Well, just to give the FULL range of thoughts..
Covenants: NO. You don't need it. Sure you can expand the stuff that you can do with labs and Covenant creation, but hey, we lived without it for quite a while, and and most of the basic 'get by' rules are in AM5.
IMO:
AM5
Then depending on WHO is asking. If you are running the game, your choices are different than playing.
Playing: The House book you will be using...
For a SG you should go in this order, depending on what kind of story you will be running:
True Lineages- This will give you a better understanding of Cannon 'Order set up'. It will show you how those main houses influence the rest of the Order. This of course includes the Tribunal meetings and some of the 'Big wigs' that run things. There is also a good run down here of the Order's Laws and description of how they work...
-If you think you will need good material next, then I suggest..
The Infernal--Plenty of 'Bad Guy' ideas here.
After those I would suggest the rest of the house books (though depending on your creativity, you might bump Infernal down below the house books)
That would be the "Bare Bones", if you are talking about extra books...otherwise, play with AM5 and save your coins...
That's actually a good point; I'm already an attorney.
Prior to True Lineages, I'd based Hermetic law on a pastiche of what I knew from reading "Female Prosecutors in Thirteenth Century England" and Edward Peters's Inquisition. Plus, of course, my education in common law.
In HoH:True Lineages, Hermetic law actually seems more Talmudic (basic principle, ruling from the tribunal, discussion), although the Quaestiores seem not as fascinated by the potential twists and turns of the code as rabbis are with the Torah.
The key issue is to choose the books that will serve you best. If you want to run a game on the cheap, I'd recommend sticking to fourth edition, and would furthermore consider using PDFs as they're cheaper. e23 allows you to get the 4e core rules for FREE, and access to many of ArM's books for relatively little money (for example, Covenants costs 15$ as a PDF, but as an MSRP of 30$). 4e books are also more likely to be found in bargain bins or used on eBay and so on.
Which books to purchase is, again, a matter of what kind of saga you'd like to play and what would benefit you. You shouldn't limit yourself to 4th Edition books even if you do play 4e - the bulk of the information can be converted fairly easily, and 5e has superior supplements overall.
Here are some recommendations. All are excellent so you should choose which to purchase based on the content.
Covenants: Excellent for the rules on customizing laboratories, which can really add variability and things-to-do for your magus' time in the lab, as well as for the rules on how to design a Covenant, which can really springboard ideas for the saga and present lots of options. It also contains detailed rules for running the covenant poltiicaly and economically, and rules for physical book quality and casting from text.
Houses of Hermes: True Lineages: If you want insight into how Hermetic Law operates, this is the best source. It also provides tools for Mercere trade (of raw vis and so on). Otherwise, unless you are particularly interested in House Bonisagus or Tremere, I wouldn't get this book - it presents an excellent version of House Tremere, but you can do without it; it presents rules for Original Research, but these are problematic and you would be better off creating House Rules off the rules of Anicent Magic, which are available for free here.
Houses of Hermes: Mystery Cults: Unlike the other HoH series, this book provides crunch that can IMHO greatly improve the saga even if you don't play these specific cults. You can pick up the strange powers and ideas and run with them to make cool NPCs, secretive cults in opposition to the PCs, and so on. I think the Merinitia section is especially useful there, presenting a host of interesting and very varied philosophies, societies, and exotic powers. As a bonus, you get excellent descriptions of four Houses, although I actually have issues with most.
The Mysteries: You can get the original The Mysteries for 4e; it is more colorful and flavorful but very poorly balanced compared to the 5e version (The Mysteries Revised Edition). Whichever version you choose, you'll receive lots of secretive societies and strange powers to brew adventures around. I don't know whether it's better to pick this one or Mystery Cults on a budget; if you are not particularly interested in the House descriptions, I think TM or TMRE is better as it presents more off-the-wall options.
Realms of Power: The Infernal: You can use it as a source of Infernal opposition, so if you like this type of story it's invaluable. I actually am not very impressed with its usability in-saga so far, but I guess I just never got into it.
The only take on Faeries out there that's detailed to some extent is Faeries: Revised Edition, for 4e. I personally found it to be too full of prose, and not very interesting. It does, however, present an interesting take on Faerie and several faerie creatures, so I'd strongly consider it for a 4e saga on a shoestring, if you're into faeries.
I wouldn't purchase Guardians of the Forest if I were on a budget given that you have The Dragon and the Bear; both are excellent tribunal books, but one tribunal book is enough.
Likewise, Ancient Magic is an excellent book, but essentially will provide you with one adventure framework so is rather expensive. Unless you engage in several, not-connected, "seeker" adventures (i.e. adventures seeking ancient magical secrets) - if that sounds like a saga you want to run, AM would be an excellent buy IMHO, as it provides both the mechanics and several excellent and interesting saga ideas, complete with minisettings, key NPCs, and whatnot.
It all depends where you want to go.
I think that if you know where you want to set your campaign, the tribunal book (if it's been wirtten or is still in print) is a brilliant jumping off point.
if any or all of your players belong to mysteries, I'd personally say the deeper mysteries of these houses are such that HoH Mystery Cults would be the most useful of the three. With lineages and Societas being more relevent if you have members of these groupings (although i take on board ealrier posters comments about hermetic Law).
I would recomend the Infernal and Divine books as sources of antagonists, but would think that City and Guild is useful to show how the 'little people' live and work (and help illustrate a rich vein of stories) Art and Academe is also very useful for outlining the accademic mindset of the era.
However in order i'd say Main rules, Tribunal Books, Lineages, Mysteries & Societas, Infernal, Divine, City & Guild, Art & Accademe, Covenants
Salve amicus,
Have you thought about sharing the costs with other players in your troup?
Where everybody buys the books that are relevant for his character and the SG the books relevant for the story (e.g a tribunal book).
So a Bonisagus Seeker could by True Linage and Ancient Magic for example.
By the way I wouldn't recommend covenants unless you plan to build up a covenant with a lot of bookkeeping. A main focus of this book is on managing a covenant with all its facets - income, cost, grogs, loyality and so on. I would first recommend one house book for the players and a tribunal book for the SG. Otherwise I agree with the previous posts. By the book the suits your saga, as they are all very good.
Widewitt
My opinion is that the Fifth Edition rulebook really does stand alone and none of the supplements are really "essential." You can have a perfectly good covenant without Covenants, or play a perfectly good Bjornaer without HoH: Mystery Cults. So as Yair said, it depends on what you want the supplements to do for your game.
There are, evidently, players who disagree with me and would not even think of playing ArM5 without True Lineages or TMRE (those are the books that seem to get the most ringing endorsements).
But to say a supplement is "essential" is really putting the cart before the horse. Supplements are ... supplemental. 
Play the game for a while, and figure out what material would most enhance your saga. Want an interesting, ready-made adventure? Calebais: the Broken Covenant fits the bill. Need villains? RoP: Infernal. Have a player who really, really wants his magus to learn the Inner Mysteries of his House? HoH: Mystery Cults. And so on...
I would agree that the Fifth Edition does "stand alone," but I think the discussion here is not about "essential" as in, "can you play without," but instead "what would make my game significantly easier to plan and run?"
Covenants is particularly good for this, as it allows for a stable framework for players to figure out the budget and schedule for improving the covenant and laboratories, not to mention balancing all the geographical/structural/magical factors. It's not essential, but duplicating its efforts is time-consuming and a frustrating process (I did it for ArM4 - and it makes covenant accounting so much easier).
This is also true for the Breakthrough rules in Hoh:True Lineages and the Mystery advancement rules in The Mysteries (and recapped in HoH:Mystery Cults). You could work out these systems for yourself, but it's a lot easier to have the playtested version.
If you plan on buying various other supplements, I've found that HoH:True Lineages and RoP:the Divine are referenced repeatedly in non-House supplements "down the line," so not having them gets a little difficult for integrating the rules of those supplements (e.g., when City and Guild refers to way a pre-fair prayer "tempers" the aura of the fair - without RoP:the Divine, there's no guidance as to what that means or the game effect).
The infernal. Good for story plots. Tha tis what really matters. Mechanics are completely secondary here
The micheling guide to the area where you are playing. Great legends and history sections. Much better than what you can get from most Ars Magica books.
One or 2 of the house sof hermes books.
I for one consider Covenants and The Mysteries to be unwelcomed rules crunching suppklements. Most people disagree with me, but I find that this detracts from a more narrative approach to Ars Magica and me and my players have generally disliked this turn int he line. They are good supplements, but not good for us and our way of playing the game.
Cheers,
Xavi
I think a good combo is