Some more books that might be of interest to people curious about Unix/Linux
internals and OS concepts in general that I forgot to put on the list
yesterday:

Unix/Linux Internals:

"IA-64 Linux Kernel: Design and Implementation" by Mosberger & Eranian
"HP-UX 11i Internals" by Cooper and Moore
"Understanding Linux Network Internals" by Bevenuti


OS Concepts:

"Operating System Concepts" by Silberschatz
Tanenbaum's trio of books: "Modern Operating Systems", "Operating Systems
 Design and Implementation", and "Distributed Operating Systems" 


This list of books and the others in the earlier email cover the range
of introductory to advanced.