My absolute two most-used books are
Programming Perl and Perl Cookbook both from O'Reilly. Programming Perl is in its 3rd edition now. I own the 2nd -- the third edition is significantly thicker and appears to be just as good as previous versions.






My most used Unix book is one that is now out of print:
An Introduction to Berkeley Unix It was my introduction to Unix, and has been worth its weight in gold through the years as a reference for scripting and commands.

Another very dog-eared favorite is C, How to Program



I have the 2nd edition (link is to the 3rd edition which looks to be even better), and use it as a reference frequently. While not strictly a Unix book, my involvement in programming has helped me grow and learn in the Unix environment too, due to the vast array of free compilers, debuggers, etc available on a Unix platform.

I have quite a few other technical books, but these are, by far, the most valuable...

[Edited by Neo on 11-29-2000 at 08:37 PM]