Use a public library.
Public libraries are a great way to save money on books. Recently, I was talking to my sister about her shopping habits and I commented that she may have to start going to shopaholic classes (joking, of course). She regularly uses coupons to save money, but I wouldn't say that she is worried about not having enough money to cover her expenses. I commented that she could save so much money by checking out books from the library instead of buying them. She rationalized buying the books because multiple people were reading them, but of course, my response back was that they too could obtain a library card and check out the books. Just a few days ago, she told me that she went to get a library card and checked out two books.
Although this story may not resonate with you, it proves the point that when you are not worried about paying for your expenses, you may be uselessly wasting money. Now, I don't mean to imply that buying your own books is wasteful, but how many times to do re-read the same book? For our kids, we re-read the same book over and over, so buying some kids' books makes sense for us, but our kiddos love going to the library to check out new books, explore the games, and spend some time together as a family.
Next time, you are thinking about spending some money on new books, check out a public library. If you are with your kids, lots of libraries have story times too. If you are in the Minneapolis area, I would highly recommend the downtown Minneapolis library for kids. It has a wonderful children's section.
Here's to savings some money and exploring the city libraries.