All in no particular order.... As a Unix systems engineer, I would choose 1. W.R. Stevens books on Unix and Tcp/ip. 2. Rochkind's Advanced UNIX Programming. 3. Most of the Unix/Linux manual pages. 4. Bash FAQ As a Unix systems programmer I would choose 1. Mythical Man Month 2. Jon Bentley's Programming Pearls. 3. K&R C and Stroustrup C++ 4. A recent Java reference and Bloch's Java Puzzlers 5. a reference and cookbook for the language of your choice -- Python, Ruby, Perl, etc. 6. Google's style guides
Also recommended in this reply