Apress ([url]http://www.apress.com[/url]) has a "Book of the Day" for $10. You get the book in PDF form only, but it's much cheaper than the $30-$40 you would pay otherwise.
For example, today's book (Feb 26) is [i]Automating Linux and Unix System Administration, Second Edition[/i].
One nice thing is that when you set-up an account, you can store your purchases and download them anytime, anywhere.