The Debian Administrator’s Handbook
This is an awesome piece of work or you may also call it a nice companion for the aspiring Linux administrator. It starts from the basics of Debian and gradually deepens one’s knowledge in field of Linux administration.
Written by two Debian developers — Raphaël Hertzog and Roland Mas, this book is available in three formats; PDF, EPUB and MOBI. It’s free to download and you may support the project through donations.