LINUX: Rute Users Tutorial and Exposition is the only Linux sysadmin's guide proven by 50,000+ Web users. It presents in-depth coverage of all aspects of system administration: user management, security, networking, Internet services, packages, config files, shell scripting, and more. It also contains detailed cross-references to LPI and RHCE certification topics, making it invaluable for exam preparation.
Reading it online at http://rute.2038bug.com/index.html.gz (or rather, as an e-book, when I get it converted into a suitable .mobi format for my shiny new toy Kindle).
This is probably rather outdated at all, but I plan to take it with a grain of salt. Already, though, the first few sections contain a much more thorough and understandable description of the basics than I have encountered anywhere else. I wonder if it's just that I finally have the prerequisites to understand this stuff, or if it's something about the writing?
Anyway, although I will probably go lighter on some of the later chapters, I'm looking forward to working my way through this. My interest is purely intellectual; I really don't want to work in IT. I just... like knowing things. I want to better understand the technology I use every day.