As a first introduction to UNIX for those already comfortable with computing, I recommend The UNIX Programming Environment, by Kernighan & Pike. It's an oldie but goodie that explains in a compact but detailed way the philosophy behind UNIX and how to approach the many tools provided with it. Brian Kernighan must surely be the best author to ever write about computing. His books are small and accessible and were mostly written before publishers discoverd that book sales are closely related to the shelf space they occupy.

This book won't make one a UNIX guru but it will give you an idea what's going on and what to look for in other books. Naturally, the link I provided will credit Mudcat with the sale.

      - Mark