Congrats Flexor! Those are a rare find nowdays!
I actually came across two in three weeks time back in high school (13 years ago). One I gave a guy $30 for a whole box of old D&D stuff, and there happened to be a MINT copy of that book in there! (He was an idiot- trying to sell off his older bro's D&D stuff he gave him for a new car stereo). The other one I found in a local game store a few weeks later, with only a dinged corner and a name written inside the cover for $40. Needless to say, I snagged it too. I've only seen one or two since then, and they were in really sad shape. Sounds like you got lucky!