This is a great opportunity to build up your teaching library at a discounted rate, or to simply get some great deals to save your students some money. Or, if you need 3 books for students, you can buy those (still saving a little money for them because you don't pay sales tax on amazon.com) and then get a free book to build up your own library. The way it works is that when you purchase 3 qualifying items, you get a 4th (of equal or lesser value) for free! Look for "Special Offers Available" on the product page (as seen below), then scroll down to make sure it qualifies for the 4-for-3 promotion.
I have gone through and found a whole bunch of method and repertoire books that qualify, but I know there are many, many more!
Music for Little Mozarts
Faber's My First Piano Adventures
Faber's Piano Adventures
Succeeding with the Masters
Bastien Piano Literature
Discovering Piano Literature (Alfred Masterworks Editions)