I put together a simple letter to enclose in a gallon size ziplock bag with their name in permanent marker, along with three books from our literacy library. I chose three books for each student based on their reading levels and topics of interest. Students were instructed to keep the bags in their backpacks whenever they were not reading their books, so they wouldn't get lost or damaged. They could return the bag to me at any point in the week and I would exchange their books for new books within a day or two. You can choose to exchange all the books once a month or twice a semester, etc. I also exchanged books when students' reading levels improved.