I polled 100+ books, because I've been playing since 1980.
Basic D&D/ Xpert D&D
1ed (all the hardbacks 75% of the modules)
2ed (99% of the hardbacks +splatbooks)
3/3.5e (so far about 90% of what's been printed, save Eberron [though it's day shall come]
I get a lot of good deals at the local...