This is the official GNOME programming manual, and is also a printed book available in stores (New Riders Publishing, ISBN 0-7357-0078-8). The book is under a free license. It's in CVS under the GGAD module. The HTML version is here
More information on the book, including errata and an outline, can be found here.
There are some translations of the book; I know several are in progress, but here are the completed translations I know about:
A tarball of the HTML version is here, suitable for browsing offline.
Find out more Application Development books at below links: