The book Palm webOS -- The Insider's Guide to Developing Applications in JavaScript using the Palm Mojo Framework by O'Reilly was published in July, well before the Palm Pre was released, meaning that the authors used an early pre-release version of the OS. This leads me to wonder if the information is current enough.
Does anyone have experience using the book for actual programming that can speak to the question if it is significantly out of date?
The book provides a decent overview of the MOJO platform, but it seems there are a few places where the code is either incomplete, or where the SDK has actually been changed since the book was submitted to O'Reilly. Your best bet would be to check out O'Reilly's errata page here: http://oreilly.com/catalog/errata.csp?isbn=9780596155254 and then reference the api docs here: http://developer.palm.com/index.php?option=com%5Fcontent&view=article&id=1654 if you have any questions about whether or not what you are reading is actually current.
The O'Reilly book is worth getting for the content and explanation. I've been learning Pre development and have found it very useful. It gives some good context around developing for the Pre. Also, they do a good job of walking you through the development of the new reader sample app, which is a pretty sophisticated app and it does a good job of demoing a lot of the feature of Mojo.
I picked up the electronic copy, along with their Javascript Book - (Javascript, the good parts). O'Reilly is offering buy 2 get one free right now on their site.