Other
Nye Thermodynamics: Basic Gas Turbine Theory
Outstanding information (and a lot of it) on gas turbine (jet) engines. Describes the history behind their development and the theory behind their operation. Well written and includes very informative graphics.
Wikimedia
Wikipedia: Jet Engine
This site from Wikipedia offers a good definition of "jet engine", then discusses the history of along with various types of jet engines. Links are provided throughout this article for additional information.