Go here to get it.
Debian is the ancestor of a large number of Linux distributions, including Knoppix and Ubuntu. It makes use of what many people consider to be the best Linux packaging system: APT. APT is very fast and makes downloading any of a few thousand programs a breeze.
Debian Lenny is the version of Linux I currently recommend for a PPC machine.
Etch 4.0r6 "Etch-n-half"
This is the current stable release of Debian, and it works very well on PowerPC, except for Airport One wireless cards.
Even though Lenny is the testing version of Debian, it's usually pretty stable. You may not want to run a server on it, but for desktops this is probably what you want to install right now.
Sid is the development version of Debian and should probably not be used by people who are not prepared for a lot of trouble. I haven't tried it.
Version: Lenny (Testing)
Finding books specifically for Debian may be hard to come by, although the following, while a few years old when Sarge went stable,
it may come in handy for newcomers (not ppc-specific, but a treatment of Debian overall). Nice that you can get it in hardcopy or pdf!
Be sure to see the author's site for publishing alternatives and updated info!