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:
Camshafts & Timings
Exhaust System Tuning
Common Rail Diesel
The list is [almost] endless.
Worth a visit