That's such a difficult question. Really, practically all the ArM5 books are superb so it's a question of what will serve your saga best.
Generally speaking, I would go with True Lineages and Mystery Cults (or wait a bit and get Societas instead) to flesh out the Order.
Given your interest in trade, however, you might want to check out City & Guild. I don't own it, so can't comment on it. Perhaps instead of Mystery Cults, which is more minor (True Lineages really gives a lot on the Order in general).
The Infernal can serve well concerning diabolic pasts, I suppose, but personally I was a bit underwhelmed by it. Never tried to put it to use, though, perhaps it plays better than it read.
You might also wish to consider Guardians of the Forest. I consider it an excellent tribunal book, and it has some adventures that you should have no problem fitting into the infernal aspects of your saga.
Ancient Magic doesn't seem like the type of book that will serve your saga well unless you mean to make Seekers a significant part of it. But only you can say what appeals to and is relevant to you
You might notice this covers a large part of the published line. It's a hard question.
I agree very much with the two other answers you've gotten - that it is a tricky question and that all the 5th edition material is worth having.
I would also recommend you to start out with the two existing house-books: True Lineages and Mystery Cults. They'll give you a very nice feel of the Order and plenty of character and story material on the individual houses. From there you can move in what direction you prefer - like others I think The Mysteries is key title to continue with.
Get City & Guild if you are going to feature trade with mundanes. True Lineages and Mystery Cults would be , as suggested , the next most beneficial.
Mystery Cults has the section for designing mundane animals and True Lineages has info on The Code of Hermes.
To me this is a truly invaluable tool because it really cuts down on, and simplifies all handling of rules / mechanics in the game, leaving you and your troupe to the most important aspect of this game (in my eyes) - the interaction of characters. (well, this depends on playstyle, but my troupe prefer roleplaying to calculation and rolling dice).
Worth to mention is that this software includes numerous references - quotes that explain virtues etc etc. from the various rulebooks, making it easy to find what you need - compared to skimming through the index pages of all your books..
I'm new to the system myself and have so far bought all the available 5. ed books. I do agree with the other posters about the importance of the two Houses books, and Mysteries: Revised. Together with the core rules and Covenants this bundle will serve as a nice foundation for your saga.
To me C&G seems like something that is very saga dependant. A good SG with some history knowledge could easily make up satisfying rules for trade, IMHO the rules regarding trade (boons&hooks) in Covenants combined with the core rules should be satisfying for most magi-oriented troupes.
Hi all. I'm also thinking about getting Ars Magica 5th edition. First, a little background: I'm a long time D&D and CoC player (still am and i enjoy it) but i like to branch out and try different stuff. I picked up the free 4th edition pdf a long, long time ago and LOVED it. Just loved it, thought it made magic actually seem like magic again (and i actually enjoyed the simple art in the book, better than a lot of b/w stuff i've seen in Runequest or HARP). Anyway, i never played Ars Magica actually (although i did port over the spell system for a cobbled version of d20 because i liked it so much).
anyway, my game group is not clamoring to play it, but i'm thinking about getting the Core book, Ancient Magic and Mystery Cults, based a lot on what i read here.
And how much better is 5th edition than 4th? thanks much
4th edition was great, but 5th is just... sublime! And I'm still to meet someone regretting getting 5th material. If you're planning on going full Ars Magica I'd warmly recommend getting True Lineages - to me, though hard to prioritize, it is the most or first supplement to the core book. IMHO that is.
I haven't read RoP:Infernal cover to cover, but my impression is positive.
Ah, you've managed to pick two supplements that, for my money, support entire sagas all in themselves. In fact, if you only had to pick one, either would do and your player magi would still have enough to pursue for the courses of their careers. Ancient Magic gives a lot of scope for Indiana Jones exploring and adventure as well as lots of wizardly lab work. Mysteries (and/or Mystery Cults) opens up not only magical strivings but also lots of politics and secret plotting. Really, really good.
The overall quality of 5th edition and all the supplements so far has been beyond my expectations. Fourth edition was good and I played with it for a good long while and I think I have just about every supplement going but 5th is a real step up.
It's bad. Really bad. It will take your desires and unspoken wishes and turn them against you, dragging you quietly down to the very depths of corruption, feeding you on deceit and malice and fear and humiliation. Not only you, but you family, your friends, all those you've ever given a damn about. It's evil like you could never have imagined. So yes, it's really good.
Cool! But what's between the covers? Does it take a Monster Manual approach, or is it more fluff such as the machination of demons behind the scenes, or rules for possession and exorcism, and stuff like that?
I have yet to encounter that approach in any Ars book - not even really the Bestiary... But it has all the other stuff you mention. And then it makes a point of addressing the generel concept, and misconceptions, of demons and devils.
Unfortunately, i have a real problem with spontaneously buying books online. I mean, i went on a splurge and bought a bunch of warhammer, HARP and Runequest stuff that i have yet to play campaigns in. Not to mention all the D&D stuff i used to buy (although i don't do that anymore, there's only so many stupid supplements they can put out until 4th edition arrives in the not-so-far future).
How does the art quality in these things stand up? Some people don't care, but the physical layout of a book is important to me. So are the rules, yes, and the quality of writing, but i am always disappointed by weak art or graphic design.