Scholastic has a program where your kids can earn up to 5 FREE books
Barnes and Noble has a program where your kids can earn a free book too! You have to get this in store - the closest ones to us are either Eugene or Portland.. So you might consider this the next time you are headed that way!
Borders had a program too last year, but they don't have any information about it yet... I'll let you know if I hear about a program for this year!
I know I am going to set my kids to work on the Scholastic ones right away :)