Is it worth purchasing Mahout in Action to get up to speed with Mahout, or are there other better sources?

Speaking as a Mahout committer and co-author of the book, I think it is worth it. ;-)

But seriously, what are you working on? Maybe we can point you to some resources.

Some aspects of Mahout are just plain hard to figure out on your own. We work hard at answering questions on the mailing list, but it can really help to have sample code and a roadmap. Without some of that, it is hard to even ask a good question.

Also a co-author here. Being "from the horse's mouth" it's probably by far the most complete write-up out there for Mahout itself. There are some good blog posts out there, and certainly plenty of good books on more generally machine learning (I like Collective Intelligence in Action as a broad light intro). user@mahout.apache.org has a few people that say they like the book FWIW, as do the book forums (http://www.manning-sandbox.com/forum.jspa?forumID=623) I think you can return the e-book if it's not quite what you wanted. It definitely has 6 chapters on clustering.

there are many parts of the book that are out of date, a version or two behind what is current. In addition, there are several mistakes within the text, particularly within the examples. this may make things a bit tricky when trying to replicate the discussed results.

Additionally, you should be aware that the most mature part of mahout, the recommender system, taste, isnt distributed. I'm not really sure why this is packaged with the rest of mahout. this is more a complaint about the software package than mahout itself.