Books! As artists we love books!
Here is a list of books I got told to read, as they were very helpful from a few friends and a lecturer. These options may relate more to concept art as that is what I am studying, but! they can relate to all art fields just fine.
They are in no particular order. A lot of these may be more realistic art, but you can easily apply the techniques to any type from Cartoon to realism. I suggest if you can find a library or a pdf to read of these books the do so! As books can be rather expensive.
1* A Theory of Fun - for game design by Raph Koster
Mainly based on game art and creation! but can be used for drawing cartoon characters and learning the basics elements of design for art for games. It brakes down the very basics and is easy to read. It also talks about what other games have that will make them successful.
It's worth a read