The one true Operating System for some people.
- Rosetta Stone - common CLI commands for most UNIX variants.
- The UNIX Wars (by Eric S. Raymond, heavily biased, of course)
- The Art of UNIX Programming (also by him, a pretty good read)
- In The Beginning... (the Bell Labs version)
- Memorable Quotes from alt.sysadmin.recovery (back when Usenet was useful).
- The Unix Haters' Handbook (for balance - whatever you think, UNIX is most certainly not perfect).