http://www.howstuffworks.com is useful for basic knowledge..
I don't know how general you are looking for or anything, but when I first first got started with engine's I bought a hayne's sport compact performance book. It was at the local autozone and pretty cheap iirc. It has some rice in it, but it does a good job explaining the basics of everything in my opinion.
Here's a link to a copy of it.. looks like it's gone up in price since I bought mine ages ago..
http://www.themotorbookstore.com/performance.html
In my opinion though you learn the most by actually doing it yourself, so go find a motor to tear apart to rebuild, or just to tear apart for the fun of it. Between multiple rebuilds and endless searches on NICO I've gotten where I am today.