And alt.religion.emacs remains a good newsgroup to learn about emacs... but only if you're willing to embrace the faith.

As far as books about emacs, the two I would suggest are:

• GNU Emacs Manual by Richard M. Stallman
• An Introduction to Programming in Emacs Lisp by Robert J. Chassell

Both of these are available through the earthly conduit of the holy fire, gnu.org. Also, as is their wont, the full texts of these books ship with emacs, along with the full Gnu Emacs Lisp Reference Manual itself: Try ESC x help i and look down the list.

The O'Reilly book Learning GNU Emacs, Second Edition might be okay, if you could find a copy (it's been out of print for ten years). Their "Writing GNU Emacs Extensions" by Bob Glickstein was also pretty good, but I think it's been out of print even longer. (It could be that O'Reilly has essentially moved on from being the uber-geek's book company... maybe it's time for Apress to do some emacs books?)

Don't forget to check the emacs wiki page: PerlLanguage.

Thou powers known to but a few, thou public API widely advertised.