This is a collection of useful links to other documents related to WebKit or WebKitGTK+.
Another Apple paper on using WebKit from Objective C (e.g. for Mac OS X or iOS) is at http://developer.apple.com/library/mac/documentation/Cocoa/Conceptual/DisplayWebContent/WebKit_DisplayWebContent.pdf. This paper contains numerous how-tos and recipes which demonstrate functionality that can also be used from WebKitGTK+.
The Hacker's guide to WebKit/GTK+ provides an overview of the WebKit source tree from a GTK perspective, along with other interesting information: http://trac.webkit.org/wiki/HackingGtk
Information on how to contribute to WebKit itself is available at http://www.webkit.org/coding/contributing.html, and links some articles on technical topics related to WebKit internals are at http://www.webkit.org/coding/technical-articles.html