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 the past two to three years, at least twelve English-language "how-to" books have been, or soon will be, published about QGIS.  There is also a book that introduces principles of GIS using QGIS (see elsewhere in the "Resources" section).  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!