I really enjoyed the book "Hadoop: The Definitive Guide by Tom White".
It has everything you need to:
a) Get started running your own cluster and writing your own MR jobs
b) Understand how to administer the cluster
c) Troubleshoot your programs
d) Learn about really important side projects like Pig, Hive, Zookeeper and HBase (of which I think Hive is the most amazing)
One thing I wish I'd done is go through the cloudera online tutorials BEFORE reading this book. If I'd done that (instead of doing so afterwards) I think I'd have got through certain sections of the book much quicker; basically I would have 'got it' quicker. See http://www.cloudera.com/resources/?type=Training
After reading the book I organized a little geek meet where I covered a synopsis of Hadoop, Pig and Hive with the development team. I also introduced them to the Cloudera training virtual machine. That is just an amazing resource for learning hadoop et al. It also introduced me to some unique cool things like the sqoop program (http://www.cloudera.com/developers/downloads/sqoop/) for reading tables out of an RDBMS like MySQL or Oracle and auto populating Hadoop and/or Hive...very useful!