A couple for 3.5/PF because they were pretty cheap as .pdf - The Complete Book of Eldritch Might, 1001 Spells for Pathfinder. I might not have gotten the former if I had been able to flip through it in the store (not commenting on the quality, was hoping for more spells and less other stuff). I would have paid for A Magical Medieval City Guide, but I think it was free.
It was 31 years old, but also picked up a used copy of Tome of Might Magic by North Pole Press for OD&D/1e/BX.