Earlier this year authors Jeff Starr and Chris Coyier closed down their affiliate program for their book Digging into WordPress. They closed it down because they discovered a good deal of fraud and abuse under the old system.
As of last Thursday the affiliate program has been relaunched, now using ClickBank and a 50% commission for book sales. For more information about the setup, and to get signed up for it, see the post on the DigWP blog.
Have you read Digging into WordPress? Would you recommend it to a fellow WordPress lover, affiliate program or not?