Great computer books
There was a very good comment on my Understanding memory usage on Linux post a couple of days ago. Besides having some insightful points about memory usage, the poster made mention of Linux Kernel Development, a book by Robert Love on the Linux 2.6 kernel. I own this book and love it; I'm not really a kernel hacker, but I have found the information in the book invaluable when it comes to understanding how Linux ticks. I highly recommend it to anyone that wants to delve into the kernel.
Although Linux Kernel Development can be read without having too much theoretical operating systems background, I would still recommend that people also pick up a good general OS book. My preference is Operating System Concepts by Silberschatz, Galvin, and Gagne, but that may be because it's the one that I used for my college OS class. I've wanted to try out Andrew Tanenbaum's Modern Operating Systems, but it's a little too spendy for me. Plus, I'd feel like I couldn't read it around other Linux geeks, what with Tanenbaum's addiction to microkernels...
My other favorite topic, besides operating systems, is networking. In this field I have two all-time favorites, one of which is Tanenbaum's Computer Networks. Not only does he cover a huge range of networking topics, but he does it in the classic hacker way -- with humor. The other favorite of mine is Computer Networking: A Top-Down Approach by Kurose and Ross. It's more accessible than Tanenbaum's book and has a nice solid, professional feel to it. Both of these books are on the high side in terms of price, but are well worth it.
Disclaimer: Greg Gagne of Operating System Concepts was my advisor in college (although, interestingly enough, I didn't know about his book until I went to graduate school). Other than that, I do not have any personal or financial ties to any of these books.