...manual of the planes, 3e version.
I've never been a very planar-oriented person, and the D&D cosmology as it stands isn't really something I enjoy a whole helluva lot. I mean, we *did* run a 2e planescape game that was alright, but in all, I preferred to stay on the Prime, thank you very much.
One of my players bought the Manual, because he was hoping to get into running his own game and thought that it would be interesting. I borrowed the book (and promptly spilled pop all over it, so I bought it off him out of guilt), and absolutely LOVED it. It is, as I like to refer to it, "an absolute mindf***".
I know a lot of people here probably love the manual, and I think it's usually regarded as one of those books everyone loves, but for me, I figured it was going to be terrible and was very pleasantly surprised to find that it's actually something that appeals to me greatly. Especially a lot of the demiplanes and planar locations that were used; I can use those in my own games, without adhering to the standard (and, to me, lame) D&D cosmology.