This is a good introduction to Emacs (though not XEmacs) covering all the basics of windows, frames, buffers, text editing, various programming modes. It touches lightly on adapting Emacs via configuration and programming.
To really update Emacs, and to cover things like e-mail, news readers, web-browsing etc. would require a thicker book. I hope that O'Reilly oblige when(if) they update the book when Emacs 22 is released.
Recommended to the beginner or someone coming back to Emacs after a break.