This week's giveaway is in the EJB and other Java EE Technologies forum. We're giving away four copies of EJB 3 in Action and have Debu Panda, Reza Rahman, Ryan Cuprak, and Michael Remijan on-line! See this thread for details.
I think you need a good working knowledge of Java to use Mahout effectively, partly because you will probably want or need to look at the source code frequently. I would not describe it as "for beginners", but at the same time I don't think you need particularly advanced knowledge of Java to use it.