Written by two Debian developers — Raphaël Hertzog and Roland Mas — the Debian Administrator's Handbook started as a translation of their French best-seller known as Cahier de l'admin Debian (published by Eyrolles). It's a fantastic resource for all users of a Debian-based distribution. Accessible to all, this book teaches the essentials to anyone who wants to become an effective and independent Debian GNU/Linux administrator.
You can get it here in the flavor of your choice : physical or ebook. Consider donating a few bucks (or even better : buy the paper version) to keep this project alive and up to date :)