As mentioned in another thread, I'm filling out my Ars Magica collection right now. I'm on a budget, and so don't want to be a true collector, simply get the most useful books. I haven't run in a while, and so while I can read a copy here or there at a friends house on game night, don't actually know how useful some of these books are. It doesn't seem like there's a thread about this, so while I have bought some books already, I think throwing everything out there would be helpful for future readers.
Edited to add: I may or may not get around to playing in my brother's game, or I may run a game of my own. I'm particularly asking from the perspective of a GM.
So, of the product list, which do you find the most useful/helpful in your games? :
Ars Magica 5th Edition Core Rulebook
The Fallen Fane LARP
The Broken Covenant of Calebais
Guardians of the Forests: The Rhine Tribunal
Houses of Hermes: True Lineages
Realms of Power: The Divine
The Mysteries, Revised Edition
Covenants
Houses of Hermes: Mystery Cults
Realms of Power: The Infernal
City & Guild
Ancient Magic
Houses of Hermes: Societates
The Lion and the Lily: The Normandy Tribunal
Art & Academe
Realms of Power: Magic
Hedge Magic, Revised Edition
Realms of Power: Faerie
Tales of Mythic Europe
Magi of Hermes
Lords of Men
The Sundered Eagle: The Theban Tribunal