Writing Apps for webOS
The PDK refers to Linux binaries that had more direct access to the underlying hardware and platform. This was commonly used in games or some media players that needed high performance graphics.
webOS Archive has restored the SDK/PDK almost entirely, including the online documentation, emulators, and other tools you'll need to get started.
Visit http://sdk.webosarchive.com to get access now!
The book is available on Google Books in its entirety, and on Amazon and most online bookstores in paperback.
Tuts+ still hosts a 5-part Introduction to webOS Development series, focused on Mojo development on the Pre. It remains a worthwhile resource for new developers.