Recently my class wrote a pigeon book (Don't Let the Pigeon Eat Candy in the style of Mo Willems) and we put in our school library at the invitation of our librarian. Other classes have been loving it and my kids are super proud of themselves.
Our fabulous librarian decided we should take this a step further and have the kids write some books independently that we can place in the library to share with others. We decided to start with writing about plants. That way their books could be as simple as, "Trees are big. Trees are green. Trees are plants." or such.
The first day in the library the kids looked at lots of books about plants and we made a list of things we learned. This mostly entailed the kids finding pictures that fascinated them and then one of us reading them the caption. We took a bunch of books back to our room and continued our research.
From our research we planned a drafted a class book and the kids began working on their own books. The draft was done on one sheet of paper with about nine boxes. The kids could list the different things about their topic and then choose an order for them. The only requirement was that their book had to be about plants. All of this took place during one week and prepared us for our next trip to the library.