One of the books that I use first thing in the year is, It's Time for School Stinky Face.
The kids love the title! lol. But it's also about a boy who has all these questions and thoughts and fears about the start of the school year. We use this book to write about some of the fears they had, or if they didn't they could make up some outrageous ones like the character did. Then they write about how their fears were conquered, or why they were "silly" fears.
This is also a good time to teach kids that their stories don't always have to be true (unless their informational writings...then that's a different story).The majority of my students (working in a school that is 86% free and reduced lunch) really don't have a lot of background knowledge or experiences on the some of the writing prompts they are given (district and state tests - again I ask you...how is testing everyone with the same test fair????), so I need to really teach them how to they can write their fictional stories with events that aren't really true - and this book is great for that. :)
Another one of my FAVORITES is the Frog and Toad series...even as a kid, these were my favorite books! I still have one from when I was like 6 years old and it's so ugly and tainted looking...I show the kids this book and they get so tickled that my book is like that, that it's so "old" and "loved."
One of the easiest ones to use for writing is Frog and Toad are Friends.
I use this one to teach how to write letters. And the kids write a letter of apology or a "thank you for being such a good friend." I practice doing it with Frog and Toad and then they write one to their friend of choice - again, a real or made-up friend.