Just finished working my way through O'Reilly's new Hadoop offering, written by Cloudera's Tom White. Overall, I recommend it very, very highly. He does an incredibly effective job of moving back and forth from specific, detailed advice (e.g. per-node memory allocation recommendations), to high-level descriptions of how MapReduce works and how to write applications which make use of it.
This is the sort of book that, at their best, O'Reilly does better than just about anyone else: full, rich info about a key piece of emerging technology.