Back when I was writing the first edition of the book, I wrote a blog post entitled Moving Targets, about the difficulties of writing in long form about a rapidly changing technical product. I'll try not to repeat myself, except to say that if everyone could petition Google to give the App Engine team a mandatory 2-month vacation, I'd appreciate it.
The plan for the 2nd edition has always been to consider everything up to the "out of preview" release to be fair game for the book. I especially want to capture features that change the emphasis and direction of how App Engine should be used. The "out of preview" release is imminent, but with monthly feature releases, the App Engine team has managed to add some big stuff in the meantime. The recent 1.5.5 release introduced the new Python 2.7 runtime environment, with multithreading, improvements to library support, and a new major version of the webapp application framework. They've also raised many of the major limits, including file size (32MB), file count (10,000), the frontend request deadline (60 seconds), and the URL fetch deadline (60 seconds). Oh, and you can now perform datastore transactions involving multiple entity groups. This was 10 days ago.
As with the 1st edition, the 2nd edition can't cover everything. But it also can't ignore the big new ideas some of these features represent. I'll be adjusting my writing schedule, and the book's release date, to accommodate some (but not all) of the recent releases, and will continue to keep an eye on the out-of-preview milestone before finalizing the 2nd edition outline. There's still some big stuff on the roadmap, but any self-contained subject about a feature currently unreleased or in "experimental" status may have to wait until the 3rd edition.
In the meantime, I'll try to increase the frequency of Rough Cuts releases, even at the risk of releasing incomplete or inaccurate text, though this probably won't start in earnest until next month. I'm also going to try posting to the blog for each RC release with a summary of what has changed.