While most of what I find useful is available through perldoc, I have copies of the first, second and third edition sitting on my shelf, and I'll likely own a copy of the fourth edition when that comes out. There are topics in the book that are simply easier to find information on by skimming the index and finding the content I need.

Besides the camel book, the two most valuable Perl books on my shelf are probably Freidel's Mastering Regular Expressions and Conway's Perl Best Practices