well.. since most characters are human or humanoid, you can't get much more important. I don't know if there are any books that discuss about how important it is, because it is so implicit. or are you just looking for resources on human anatomy..
if you're doing some kind of thesis, you may have to just look for prime examples of character design with strong anatomy and point out how weak they would be with incorrect rendering of the human form. if thats the essay subject though, it's not very compelling. its not like the question you're asking could go either way, the answer is always going to be 'very'