I noticed the other day that O'Reilly has a new book format for
prerelease software that relies on planning for and making
incremental updates as the software is developed.  After the
software is released, the prerelease book and all of its updates
are cleaned up and turned back into a book with standard
formatting.  It looks like a Silverlight 2 book is being used as
the test subject.  I think this approach is interesting as a
response to how quickly software is developed and then obsoleted
these days.  Additionally, software is being developed with
increasing amounts of transparency.  Customers get early builds
and trickles of information long before the official release
date, and often while the software is still very malleable.  By
the time software is stable enough to write a real book about
it, you've already missed the best window for publication for
that release.