As a player of a spellcaster in a banewarrens game I'd say Book of Eldritch Might I as the DM has that and BOEM II and made them available for learning and acquisition in the game. I've gained many spells and even a feat from BoEM I and spells from BOEM II, although my character also has material from FRCS, Magic of Faerun, Quintessential Wizard, and Oriental Adventures. I was cursing myself in the last game for not bringing OA to have the exact effects of stuff on hand.
The DM is also using the counters I got from Dragonscale counters a lot.
The other PC is playing a paladin and he uses stuff from Domains of the Faithful, Call of Duty and Forgotten Heroes: Paladins.