While slowly putting together my ArM5 library (was this supposed to be a LARP of library building in ArM5?) I occasionally run across a cheap ArM4 book. I picked up Triamore this way.
What I am wanting to know is if you have one or two books you still regularly use in your ArM5 games. Even if just for the setting or characters or storyline, what have you found most useful from the previous edition in your current edition game.
I still use Heirs to Merlin when I'm running my Stonehenge saga, and The Medieval Bestiary, Revised when I need to stat critters or come up with Heartbeast stats.
I depend heavilly on the WGRE (Wizard's Grimoire Revised Edition) for information on Hermetic culture and the Periphrial Code. I also use ToH-Iberia (ArM3), both for setting inspiration and because of the original Flambeau origin. And it is the best Tribunal book yet written.
I use tribunal books, mainly to fill in the holes-- although I drop the more demon-y bits when needed. So books that see reference are, Dragon & the Bear, Heirs to Merlin, Blood & Sand (although not all of it, only the parts that aren't superceded by Cradle & Crescent), ToH: Iberia, ToH: Rome, Sanctuary of Ice. And then I'm using them mainly for NPCs and notable geography, some tribunal history/code/intrigues.
I do mostly what everyone else here: my saga is based on the Tribunal of Thebes, so what I mostly use from older editions are Blood & Sand (as said, where not superceded by tC&tC), ToH: Rome (for intrigues from venetian magi) and The Dragon & The Bear (for some minor subplots regarding mongols). I also make heavy use of Medieval Tapestry (4th) and Mythic Europe (3rd). I've made extensive use of both Myhic Places books, but I've alerady squeezed all there was in them so I don't tap into them anymore. I plan to use South of the Sun and The Sorceror's Slave in the future, but I'm not using them currently. I also occasionally plunder the Medieval Bestiary, but mainly for critters' concepts while fully redoing the stats from scratch for 5th.