I just got this in an email from Barnes and Noble:
Kids earn a free book after reading 8 books, filling it out on a form the site provides, and turning it in. You have to turn it in at a store location. The site even gives "staff picks" by grade for suggestions on good books.
The form breaks down grades 1&2, 3&4, and 5&6 and the books you could earn in each grade group.