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 GNU/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 GNU/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: GNU/Linux System Administration - training.gbdirect.co.uk
Around 169 pages: Introduction to the Command Line - Free Software Foundation
Around 396 pages: Two Bits - The cultural significance of free software - by Christopher M. Kelty
Around 522 pages: The GNU/Linux Command Line - William E. Shotts, Jr. - EXCELLENT GUIDE
An Introduction-Overview of Unity in Ubuntu 12.04 for Beginners (Video)
Cheat Sheet containing most commonly used commands in GNU/Linux
Free books for advanced GNU/Linux system administrators
Around 329 pages: GNU/Linux From Scratch PDF - by Gerard Beekmans
Around 737 pages: System administration books written by Paul Cobbaut, a very experienced trainer
FreeDesktop - Official X.Org compose key sequences. Use this as reference to FreeDesktop (used as the default in many systems).
Paperback (Not free)