Together with Simon St.Laurent and Joe Johnston, Edd Dumbill wrote a book on XML-RPC for O'Reilly and Associates on XML-RPC. It features a rather fetching jellyfish on the cover.
Complete details of the book are available from O'Reilly's web site.
Edd is responsible for the chapter on PHP, which includes a worked example of creating a forum server, and hooking it up the O'Reilly's Meerkat service in order to allow commenting on news stories from around the Web.
If you've benefited from the effort that has been put into writing this software, then please consider buying the book!