Simon Cozens loaded his book Beginning Perl into MediaWiki. It's already online for free, and it's also still in Apress's catalog as a second edition by James Lee. There's not much information about the wiki yet, but I was able to create an account, so I suspect Simon might welcome contributions and help, at least on his already-published books.
Along with that, he's also loaded the material for a three-hour Perl internals training course. This isn't the same thing as Simon's Extending and Emdedding Perl , but it still looks like some hard-core stuff for internals hackers. As internals change, I suspect Simon (and others) can now keep that information up to date.
He has another link for a yet-untitled Maypole book under the category "Books in Progress". No further details on that yet, though.