This covered material really should centrally located. It is slightly unforgivable that a company like Zend doesn't offer the needed documentation.
Please, feel free to encourage Zend to submit better API docs to the PHP doc team (but do it nicely (-; ). We'd love to see them. What we have now is old, and far from complete.
Historically, the problem has been that those with a good understanding of internals don't have much time for docs, and those with time/dedication to docs don't have much understanding of internals.
FWIW, I second Wez' opinion about George's book: it's a very good read, and it focuses on things-internals in the late chapters.
S