A great place for free online books is Linuxtopia.org. The site is a collection of about 60 online books including Linux system administration guides, beginner books and programming and scripting tutorials (C, C++, Java Shell Scritping, Perl etc). The site also has How To guides that help you do basic Linux admistrative tasks.

You can also search the entire library to find an answer to a question which is also quite useful. The site has about 10,000 pages information so most answers are in there somewhere.

The list of online books is available at: http://www.linuxtopia.org/online_books/index.html