In a note submitted by the folks over at PHPClasses.org, there's details on a change in their book review section. It's no longer just limited to the reviewers the site picks out. Instead, anyone can submit a book review to the site on their choice of PHP book.
Writing books often does not pay enough to compensate the time and skill it takes. Many authors write one book and then move on working on paid consulting as it often pays much better. But writing good books also gets authors good reputation.
The general idea of this effort is to promote more good PHP books thus encouraging more authors out there to get up and get out and write their own quality resources.