If you want to learn how to draw hats -- y'know...those 1940's detective-style fedoras that you see in the movies -- then take a look at these scans from a couple of old drawing books...
Old-school fedora hats can be kind of tricky to draw...and we have to draw a lot of them on Phineas and Ferb. Secret agent Perry the Platypus wears his fedora hat in every episode, so we end up having to draw hats from all kinds of angles a few dozen times in every storyboard.
One big reason that "how to draw hats" reference is so helpful is that you just don't see fedora hats very often these days. There's lots of great hat images in classic Bogart movies and other Film Noirs. I just watched The Big Heat a few weeks ago, and I couldn't stop looking at Glenn Ford's grey fedora (well, it was a black and white movie, so everything's grey...).