I have an even dozen, roughly sorted in order of usefulness. Note that the books at the bottom of the list are still useful, they just have a smaller domain. A lot of people rave about Effective Perl Programming, but I think it loses a few points on its index -- it's not very good. A number of times I wanted to re-read a particular passage, and I kept drawing a blank, so now it's been swapped out, until I have time to read it from cover to cover, this time annotating it in the margin.