One of my favorite books on the topic is Eats, Shoots & Leaves: The Zero Tolerance Approach to Punctuation by Lynn Truss. This is a very fun and pithy guide to proper usage of English grammar. But even in this book, Lynn talks about James Thurber, who wrote for New Yorker editor Harold Ross in the '30s and '40s. They had extreme opposite views on how the comma should be used:
According to Thurber's account of the matter (in Years with Ross), Ross's 'clarification complex' tended to run somewhat to the extreme: he seemed to believe there was no limit to the amount of clarification you could achieve if you just kept adding commas. Thurber, by self-appointed virtuous contrast, saw commas as so many upturned office chairs unhelpfully hurled down the wide-open corridor of readability. And so they endlessly disagreed.
I don't know about you, but I find this bit of information highly comforting. Though there are some absolutes when it comes to grammar, there's clearly a bit of subjectivity as well. Whew!
I considered elaborating on those "absolutes", but it would take way too much time to type them all out. So instead, I'm going to offer my advice: 1) Pick up Eats, Shoots and Leaves - it's a fun and informative read, and you'll never look at a sentence the same way again! 2) Pay attention to the specific publisher(s) you're aiming to write for. When you're reading books by their authors, take note of how they use commas; do they use semicolons, etc...?
Now I know many might say, "But that's what copyeditors are for! Why do I need to know grammar?" Well, maybe you don't, but I do think submitting the most polished version of your work to agents/editors is a good idea. It will make you look more professional and hopefully make the agents/editors take you more seriously. Will it land you a contract? That I can't guarantee! LOL!