Aside from the Core Rules (being the Aedon Heroes' Lorebook, Veiled Lorebook, and Bestiary

)...
Hmmm... On a page count vs pages used ratio, I'm going to go with
Alchemy & Herbalists (Bastion). Great material, helps fill in some of the "gaps" left from lowering the magic level. Books that contain rules that we have adopted and frequently use include
Relics & Rituals (S&S Studios),
Occult Lore (Atlas),
Nyambe (Atlas), and each of the
Master Class books (Green Ronin, except for
Noble's, which I haven't gotten yet). Also, weapons from
Arms & Armor (Bastion) make frequent appearances, but they've been "culturally assigned", so what and when is very situation dependant.
Toolbox,
War,
Mercenaries, and
Magic (all by AEG) also have a bit of material we use often, as does
Demonology and
Elementalism (Mongoose). Stuff from the
Book of Erotic Fantasy (Valar) is also starting to make appearances and will be used regularly.
We also recently did our last batch of one-nighter playtesting in an Egyptian-based region, so
Necropolis (Necromancer) got a lot of use about two months ago. As the region does exist in the campaign, I'm fairly sure that book will see more use in the future.
We want to use
Fields of Blood (Eden), but the OGC declaration is a bit off, so we might pass on the system entirely (unless they devise some form of Trademark License akin to M&M's Superlink, which I'd be all good with)...
Future purchases we expect to get a lot of value from:
Into the Green and
Into the Black (both by Bastion, plus
Arena if the
Fields of Blood thing doesn't pan out),
Empire (AEG),
Mesopotamia (Necromancer) and
OGL Ancients (Mongoose).