Independent guides
These guides are independently maintained. Much of their content is included in the Ubuntu community help pages, but newer material in these guides may not be.
Ubuntuguide - Comprehensive guide for Ubuntu, since 2005.
Kubuntuguide - Comprehensive guide for Kubuntu, since 2007.
Ubuntu Geek - a wealth of information, but in blog format
Psychocats' Ubuntu Linux Resources - a collection of random tutorials for Ubuntu
Ubuntu Sharing - tips in blog format
The Owlcroft Ubuntu Help & Advice Pages - from choosing through maintaining
How do I pre-install Ubuntu for someone else (= OEM install)?
Free guides and free manuals for Linux beginners
Around 26 pages: The Cathedral and the Bazaar - Eric S. Raymond
Around 32 pages - Ubuntu_Beginners_Guide_-_MakeUseOf.com.pdf - Courtney Loo
Around 126 pages: Getting Started with Ubuntu 12.10 - The Ubuntu Manual Team
Around 130 pages: Ubuntu 8.04: Ubuntu Pocket Guide and Reference - Keir Thomas
Around 147 pages: Linux System Administration - training.gbdirect.co.uk
Around 396 pages: Two Bits - The cultural significance of free software - by Christopher M. Kelty
Around 522 pages: The Linux Command Line - William E. Shotts, Jr.
an-introduction-overview-of-unity-in-ubuntu-12.04-for-beginners
Linux Cheat Sheet containing most commonly used commands in Linux
Free books for advanced Linux system administrators
Around 329 pages: Linux From Scratch PDF - by Gerard Beekmans
Around 456 pages: Self-service Linux – Mastering the art of problem determination - Bruce Perens
Around 737 pages: System admin books written by Paul Cobbaut, a very experienced trainer
Linux software equivalent to Windows software