Beginner/Intermediate books on Linux?

I've been a Linode user for almost 3 years now. When I started out, I had a very limited knowledge of Linux and system administration in general. However, with the help of Linode Library and loads of online tutorials, I can now say that I am somewhat "advanced" beginner. That specifically means that I am able to setup a LAMP/LEMP stack, deploy Rails applications, configure and maintain a mail server (Postfix + Dovecot), etc.

Now I'd like to get some proper education on Linux and system administration in general. What books do you recommend? I need some basic introductions, but not really basic, as well as intermediate level books. What's the best way to expand my knowledge? I have a few dozens of bucks to spare on Kindle books, so please, fire up your suggestions. :-)