I have recently skimmed through a book published by Packt Publishing on Performance Testing with JMeter 2.9. Since I have used this piece of software for a while for ArcGIS Server tests, I thought it might be worth reading a book to get a deeper understanding of some basic concepts related to testing per se.
I think the book is very well structured, the amount of information is just enough to get anyone started yet you are not overwhelmed by unnecessary details and minor things that usually make it hard to read reference books on programming. I liked very much that the book has lots of step-by-step tutorials for each chapter where one can test the things described and shown on figures on his own. Another useful thing is that you get instructions on how to set up the JMeter including configuring the Java settings. The last chapter, Helpful Tips, is something any user of the JMeter would benefit on, and it seems as in this chapter one can get more helpful tips than on the Internet (blogs and web sites). The book is not supposed to replace the official software documentation from Apache yet is really good as a kick-off start place to get you on track with the JMeter and see some examples how you can benefit from using this software.