Usually old medical books on anatomy are a good place to start, specifically the ones with lots of pictures. If you're not sure which ones to grab, I suggest visiting your local library or book store and flipping through a few of their books there. If you find a good one, either check it out or buy it.
There really isn't a single book that works the best, as any anatomy book with lots of pictures will do, especially skeletal and musculature pictures so you know what the skin is wrapped around and conforming to.
Alternatively, you could check around your local community colleges and see if they offer some kind of Life Drawing class. I want to say most community colleges will let you sign up for only a few classes, and tuition is much cheaper than a full blown art school. Besides, learning from an art teacher is a lot more effective than learning on your own.
Another alternative, YouTube has hundreds of people who film themselves whil they draw/paint/digitally paint/exc., just take a quick look through some of them, and I'm sure you'll find a few explaining how to do some basic character drawings.
