Advanced Dungeons & Dragons Player's Handbook, 2nd Edition

The don't-run-a-session-without-it DMG, at least, until you're entirely familiar with its contents. As with the PHB, I read this from cover-to-cover too. The role-playing and world-building stuff could've been much more, so the book ended up as mostly being used for the treasure tables once you're familiar with most of the rules clarifications in here. Four stars for continuing and improving on D&D, -1 star for not improving on the RP aspect.