C.M.
I don't think a first grader needs a book on how to write stories. I think they should be given a journal and allowed to fill it up with pictures and writings of whatever they want.
A first grader can barely write or spell. They are working on the basics of sentence structure, grammar, paragraph structure, etc.
I would say the best option is to read stories to your first grader and have them read to you. Then have them dictate stories to you in their own words. Let them be as creative and free as they want. There's plenty of time later on to work on improving writing skills.