Handbook of Model Rocketry (6th Edition) by G. Harry Stine.
I was delighted by how easy it is to read! It is an excellent source for beginning rocketeers, and has a lot of more detailed information for those wishing to delve further into the hobby. I learned several new things just in the opening chapters.
There are chapters on the basics: Getting Started, Rocket Construction, Rocket Motors, Ignition System, Launchers and Launching. More advanced topics follow in later chapters: Equations for determining maximum altitude of a particular rocket, Aerodynamics, Stability, and Multi-stage Rockets.
The book also includes the source code for BASIC-language computer programs such as a simulation of rocket flight given certain characteristics.
There is a wealth of information in this very readable book, and I believe the first six chapters should be required reading for anyone intending to launch model rockets.
This edition was published in 1994, but there is a newer 7th Edition published in 2004 available at Amazon. While much of the technology of rocket building and flying has remained unchanged, this new edition does include information on a lot of the developments in the ten years that passed (microelectronics, new engine types, sources on the internet). It also includes a new chapter on High-Power Rocketry! I think I'll pick this version up, too.