Warren Buffets letters to shareholders would be a good place to start. Read all of them.

The intelligent investor and securities analysis by Ben Graham are essentially reading albeit abit dry.

I'd also recommend (when looking at equities) that you do lots and lots and lots of research on individual stocks and really get to know them before you take the plunge and fully understand why you've decided to buy.

Also, have an entrance and exit point on all investments. Unless you're going for the 'value investing' strategy buying and holding forever.