I thought that maybe it would be good with a thread that everybody could use, when you get a brainfreeze and don't remember where and in which book you can find specific information.
I can start:
I recall seeing a spell guidline for Mentem spells for spells to recollect old memories. I just dont remember which book and page it was.
I think that the base level for the effect was 1-3 but I am not sure.
the spell using it is in HoH: TL, under house Guernicus.
The CrMe Guideline is on HoH:TL p.72f, the spell By His Works on p.73.
Cheers
Thank you One Shot!
I tried to locate it in Arts & Academic and Guild books. Never thought that it should be in the HoH:TL.
Guidelines not in the Ars Magica 5th Edition text can be found here. It hasn't been updated in a year, and I'm unsure if any new guidelines have been added in that time, in any event.
I know there is a special rule how to design a spell to work better in a Divine, Faerie or Infernal Aura but can't find it. Can someone point me to it?
You mean the Realm aspected spells given as an example of original research in HoH: TL (Bonisagus chapter)?
Sorry, can't help you much there 
Thanks, yes HoH:TL 34 is what I was looking for. Sadly it turns out I remembered wrong and it don`t work with the Divine.
Where can I find a thread with ideas for experimentation results (in the "side effect" roll)?
Where can I find a good hint on what a summa in Parma amgica and other skill would cost (in game, my players want to buy one) of average 6-8 quality
You may have also seen something with the Hedge Magic (Revised Edition) book where Learned Magicians can get the 'Entreat the Powers' Virtue that lets them mitigate the effects of other auras -- including the Divine. (HM:RE pp. 87-88)
Yes, but only Natural Magician can be purchased by anothers, i saw this virtue like Craft Amulet, Spernatural but easily like a Supernatural/Hemetic Virtue. How get that kind of things are possible if you see the first chapter of LoM.
Covenants has some suggestions for this on p. 95 (well, the entire Library chapter is relevant, really).
It should be noted that
- The price categories assume you are using the rules in Covenants for increasing Quality. If you do not, reduce the quality of each category by 3. These categories are also somewhat disputed - if you look around this forum, you will find several discussions of what qualities of books are likely to be available, and in what sort of quantities.
- Having played around with these rules for some years now, I feel that book prices (in Covenants) are probably too low. This may be a matter of taste, but i general, a covenant cannot afford to allow members to study from Vis (as opposoed to buying books) at the rates given in Covenants. Equally, I find it problematic to make the economy of copying books does not hold together, unless it is assumed that every covenant employes 1 or more non-Gifted scribes to copy out every tome avaiable.
- Some troupes have issues with books on exactly Parma Magica, considering it a Secret of the Order of Hermes, worth killing for. Which is consistent with how it has been presented throughout the lifetime of the game I think. So probably no books on Parma Magica, except possibly as story hooks, though it really depends on how paranoid you see the OoH as being.
Now if only the order adopted a mystery cult like approach to Parma 
Because I forgot:
Guardians of the Forest has Durenmar, where you can usually trade for books by offering books, or by offering you own time (copying books) in return for access to the library.
I have used this system less, but so far it appears more balanced.
How would this help?
It would still be an Arcane Ability, not a lot of restrictions on learning those.
It's not the Order that would need to adopt it as a Mystery but the rules.
Even if it was formally a Supernatural Ability, those can be learned by those who have the Gift, albeit at a penalty.
And that means books on the subject (good ones anyway) are still a very real threat to security (through obscurity).
Remember the example of the Verditius Mystery of Automaton Magic?
More the fact that cult lore lets you use everyday words to encode a message, so you could have books on parma that are also on magic theory, and unless you know the cult lore, you can only see the magic theory text, effectively.
I missed that bit. Could you point me to a page reference please?
I've been wanting it to do that for a while
I'd like that also but I fear you might need to start a thread of its own.
W
TM:RE Page 11, first box out. "Two magi with this ability for the same Mystery Cult may well be able to hold a seemingly innocuous conversation without outsiders realizing that a second, secret, conversation is hidden within the innocent phrases."