This is thick, though not necessarily physically so: very small type written in language that is very hard to read linearly. That said, this tome is absolutely FILLED with bits of gaming wisdom.
I've never actually been able to pick this up and read it from start to finish due to the dry, rambling language used. I've skimmed through it though more times than I can count, going over some sections more than others.
This combination of reading difficulty mixed with my only being able to skim the material has given the book an almost magical quality: I've found that I can pretty reliably pick the book up, open it to a section I'm interested in reading about, and be almost assured that I'll read something I haven't read before (or don't remember reading). Quite frequently this "new" section will have some pearl of wisdom embedded in it that I've never seen before. I'd swear Gygax somehow cast a spell on the book ("Book of Infinite Wisdom"?).
Also, the Appendices are a gold mine. TONS of tables mixed with information that any fan of these types of games would love (Appendix N, looking at you).
I easily rate this 5 stars as a book that I love and hold dear. But as a gaming reference I find this a bit harder to rate. I know I could bookmark the sections I need to know, but part of the charm of playing this version of D&D is in how loose it plays. Why bother then?