Here’s a site that talks about the rules of comma usage: http://owl.english.purdue.edu/handouts/print/grammar/g_comma.html
Here’s a comprehensive guide to English Grammar and Style: http://andromeda.rutgers.edu/~jlynch/Writing/contents.html
http://www.usingenglish.com/glossary.html This is a comprehensive glossary containing the explanations of about 250 English Grammar terms.
http://en.wikipedia.org/wiki/Copy_editing The above Wiki link gives an insight into the art of copyediting.
Here’s the link to one of the most revered authorities on English Grammar. http://www.bartleby.com/141/ It’s the “Elements of Style” by Strunk and White.