Do you need to understand good grammar and punctuation to write a book??
As a new writer, I can assure you that you don't. If you worry about whether it's a comma, semi-colon, or colon that's needed for anyone to understand the story, then you'll never get beyond the first page. Just get down the words on the 'paper'. Thank God publishers have blessed authors with great proof readers.
When I started writing, I worried too much about italics, brackets, ellipsis, commas, and a list a mile long of rules ... and not enough about the story.
Despite everything I've written above, I gave in and bought a book yesterday 'My Grammar and I'???
One thing is for sure. It's NOT going to make me stop writing, but it's probably worth understanding some of the rules of the game.