I buy programming books on subjects peripherally related to what I do when they go on sale and then read them later when I have time. When Codys went out of business last year I was able to get an OCaml book and a half dozen odd graphics books for a few dollars. I still haven't found the time to read them thoroughly. That said, I just ordered
http://www.lua.org/gems since I do a lot with Lua these days.