I really don't know, but I can take wild guesses.
Things are changing fast. Hadoop is an excellent tool for its purpose and is actually getting somewhat mature. It is not an ideal tool for machine learning algorithms. I would not be surprised if some of the other distributed computing frameworks that are emerging, which are designed for a bit more general purpose application, become more popular within a few years for stuff like this. But that's still a few years off at least.
And if that changes, I would not be surprised if Mahout (or another project) changes to reimplement on another framework.
For now I think Mahout has figured out its identity: clustering, classification, collaborative filtering on top of Hadoop. It implements a lot of stuff, and in my opinion has a fair bit of work to do to polish and document what's there. I do not anticipate big changes in what it does, but I do anticipate refinement.
There are no plans for a second edition of the book at this point, as it would be years away at least. The final version of the book is written for Mahout 0.5, which is recent as of a few months ago, and that should remain a useful guide for versions of Mahout for the next 1-2 years.