One way to encourage summer reading is to find out what preschool programs are available at your local library. Call, consult the website, or better yet, visit your library to check out the programs that are designed for preschoolers. Trained children's librarians will choose books that will engage and entertain very young children. These events are free, air-conditioned, and will be a fun activity for you, or another caretaker, and your preschooler.
Summer = Reading. Pass it on.