Recently I've come across this very nice Debian manual describing the famous open-source linux-based distribution.
The handbook was written by 2 members of the Debian project Raphaël Hertzog and Roland Mas which is a good sign generally as the guys must know pretty well what they're writing about. The book is aimed to cover all the aspects of Debian, including how the distro is build along with the practical aspects of administering a Debian based system. It is also a fresh book, published on May 2012 (not in 2001 for example).
In short, if you want to learn about Debian from within, the book is for you. If you plan to administer Debian based desktops or web-servers, the book is for you as it covers a lot of topics a future system administrator should know:
- how to set up the Apache webserver on Debian,
- how to secure a web-server on Debian,
- virtualization on Debian (KVM/Xen/LXC),
- automatic installations, etc.
You can get an electronic version of the book from here for free, also check their website for ways to obtain a paper-copy if you prefer books in that format.