Let's see. D&D books I don't use... hmmm....
Well, recently (last 3 years or so) all of them. Still own them, and look at them for ideas now and then, for whatever reasons I might have at the time, but I don't "use" them.
When I actually played D&D.... I didn't ever use PsiHB, DMG, MM, MMII, A&EG, SS, MotP, D&DG, any of the class "splats", OA, and a bunch of others. Of course, I own them, but, again, I didn't "use" them. And, here, I'm talking about WotC books only. They had a taste of "blah" to me. All stuff I'd seen before. Was the same way with ToH by NG, too. I don't use that book, but I own it.