Unlike many open source products, Quantum GIS (QGIS) is blessed with three comprehensive, well-edited, and printable manuals (User Guide, Training Manual, and PyQGIS Cookbook) in the form of PDF files downloadable from the QGIS website. In recent years, at least three English-language books were written about QGIS. In addition to these publications, there is a vast collection of tutorials and "how-to" documents, available as PDF files, web pages, or web-based videos, that are accessible through the Internet. A dedicated group of prolific bloggers also contribute to the growing body of "how-to" tutorials. Finally, two active user forums provide support by the large user community to their fellows, and one archived forum also provides past questions and answers for reference. This diversity of information sources reflects and meets the needs of the growing base of QGIS users. This section is a partial compilation of the various English-language resources concerned with QGIS. Let me know if you have any others to add!
Before I introduce books on QGIS, let me introduce you to a book by Gary Sherman, which will help you if you need a quick introduction to Geographical Information Systems (GIS), and in particular the use of open source tools in the subject area.
The book is The Geospatial Desktop: Open Source GIS & Mapping (2012, Locate Press). Gary Sherman is the founder and original developer of the QGIS project. It teaches desktop GIS using open source tools. It is an update of a 2008 book entitled Desktop GIS: Mapping the Planet with Open Source Tools, now out of print.
While the first chapters of the book offer a brief introduction to the subject area of GIS, note that in a GIS curriculum in universities, each of the subject areas is actually covered by a full term (semester or quarter) course. This book is available in paperback and PDF versions.
The Quantum GIS Training Manual (2013, Locate Press) by Tim Sutton et al. is a product of years of teaching experience by Tim and his colleagues through their consulting GIS firm in South Africa. Tim is also a prolific blogger on the subject of QGIS and related technologies. His blogsite (see link on his name above) is chock full of video tutorials and publications teaching QGIS.
The Quantum GIS Training Manual uses QGIS v1.8 for its lessons and tutorials. Even though the current version of QGIS is 2.x, this book remains extremely useful in illustrating basic and advanced uses of QGIS for various cartographic, database, and spatial analysis tasks.
Learning QGIS 2.0 is probably the first QGIS book that covers the newly-released QGIS 2.x versions. Many, many improvements have been incorporated into QGIS 2.x over the 1.x versions, and this book introduces many of the key changes as well as introducing the software to beginning users.
Published in September 2013 by Packt Publishing, is by Anita Graser of the "Free and Open Source GIS Ramblings" blog fame (see blog list below). She is a QGIS developer, as well as a prolific writer of QGIS related tutorials and blog articles. She has also contributed to numerous GIS-related columns and websites.
Through the publisher, the book is available in a paperback version as well as in several electronic formats.
One of the wonderful things about QGIS is that it is programmable! If you are the kind of person who can't leave well enough alone and just loves automating processes by writing macros, plug-ins, or even changing the code (QGIS is an open source project, after all), then this book (as well as the official QGIS document below) may be for you!
The PyQGIS Programmer's Guide (2014, Locate Press) by none other than Gary Sherman is available in paperback and PDF versions.