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.
Living up to their free software ideals, the book is freely available (that is under the terms of a license compatible with the Debian Free Software Guidelines of course).
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