If you r kids are in public school, you are probably very familiar with Scholastic Book Orders. I have volunteered the last few years to handle the book orders for both of my children's classrooms, so I have learned a lot about the program.
Scholastic is a great resource for inexpensive reading material for your children. You can often find popular books up to 50% off retail price. The added bonus to ordering thru this program is that you earn reading materials for your child's classroom. I have seen the wonderful number of free books that come in and am constantly in awe at the generosity of this company.
I am also now the "Book Fair Lady" at my children's school and this is also a wonderful program. You don't receive the discounts you do thru the order forms, but the earnings by the school are even greater.
You should also check to see if there is a warehouse in your area. They have special sales a few times each year and you can go shop right from their warehouse and the savings there are astronomical.
Scholastic also has a wonderful website loaded with wonderful resources. I hope you take a moment to check it out.