This website is a valuable source of information on Engine matters. It is a compendium of stuff that teaches you how engines work and how to get the best out of them.
Numerous articles can be downloaded (pdf format requires Adobe reader or similar (download
here)).
The full colour articles cover topics such as:
TurboCharger Efficiency
Compressor
Fuel pressure
Camshafts & Timings
Engine management
Exhaust System Tuning
Turbos
Mapping
DYNOs
Compression
Fuel Injection
Brakes
Common Rail Diesel
Instrumentation
DPFs
The list is [almost] endless.
Worth a visit