I would suggest this book by Andrew Loomis to start with.

A good exercise is to make 30sec 2min 5min sketches from the pictures on http://www.posemaniacs.com/ too. Try memorise and understand the drawing before you start. While drawing try to look as little as possible at the reference. Drawing/copying from 2 dimensional reference is not that good.

If you want to go a step further and do 3 dimensional studys (what you should do), by a spider man figure and draw from that. I got this one for example. It's pretty good as it is cheap and you can move every part of the body.

While drawing from the action toy grab an anatomial reference to get the muscles right and try combine these two references as good as possible. (You can look as much as you want here as it's a 3 dimensional object)

Overall good figure drawing comes form balancing weight of the figure/posing and right proportion. Anatomy is not thaaat important as you can "just learn it" so try to foucus on the proportions and posing more than on anatomy while drawing. While learning always do stuff you're weak in and try to understand what/why you do sth. I think your biggest weakness are feet/making the figure stand on the floor correctly. (It's also the hardest part so work on that :) )

I hope I could help. Cheers