Hi Diddy,
The previous posts in reply to your question are all good and accurate. And I would endorse the mentioned book if you have a general interest. One general comment I will add is that many questions that took a lot of time and research to find answers to only a few years ago can now be answered quickly and accurately thanks to Wikipedia. With respect to your question in particular, this Wiki page is quite helpful:
Density altitude - Wikipedia, the free encyclopedia
Grizzled