While brushing up on my Logical Positivism, and ended up back at Wikipedia. Wikipedia is a free online encyclopedia. Reading Wikipedia takes me back to my childhood when I would spend hours reading through our ancient encyclopedias. The particular books we owned (I forget the brand), had a very reductionist bent. That must be where I get my empiricism from!
Anyway, I highly recommend Wikipedia. The WikiMedia project also has a free dictionary and a collection of free textbooks and manuals. Most WikiMedia resources are available in multiple languages.