I found a copy of the 1991 D&D Rules Cyclopedia at a yard sale today for $1! I also found the D&D Thunder Rift module, the D&D Creature Crucible PC4 and the D&D Dawn of the Emperors: Thyatis and Alphatia boxed set, also all for $1 each. It was the best gaming book yard sale day ever!
If you are so inclined, I talk about them a bit in my latest video.