"Introduction to GCC for GNU Compilers gcc and g++", by Brian Gough, Foreword by Richard M. Stallman. (2004).
This book is a good read with an emphasis on simple but relevant content. The book is published under the GNU Free Documentation License, and copy can be found here.
Information on purchasing a copy can be found at http://www.network-theory.co.uk/gcc/intro/. Since part of the purchase price goes to raise money to improve and create more free software and documentation, I purchased a copy to read on the train. The book is small enough to fit inside any cargo pants side pockets.
You may also be interested in the following book:
"Comparing and Merging Files with GNU diff and patch; for Diffutils 2.8.1 and patch 2.54", by David MacKenzie, Paul Eggert, Richard Stallman. Edited for publication by Brian Gough. (2002).
If the above free download doesn't work, since I'm not sure it will work outside the United States, try the following Souptonuts downloads