1) d20 Call of Cthulhu. I haven't played this in years, but I still read portions of it from time to time. The GMing advice is among the best ever written, and the rest of the book ain't nuthin' to sneeze at neither.
2) Monsternomicon. The first monster book that was easy to pick up and read cover to cover, and which, incidentally, I've done more than once.
3) 3e Unearthed Arcana. Somewhere within its pages is a solution to almost every concievable problem with 3.5. Plus, plenty of new options to just play around with.
4) 3e Manual of the Planes. I loved how this book was presented as a bunch of elements for kitbashers to play with. Pretty much everything in it is modular, and well done.
5) Fiendish Codex 1: Hordes of the Abyss. My favorite pseudo-setting book to read. Tons of great ideas all through this volume.