One big thing to save money on books is not to buy them new, ever. It just seems like that almost any book you want can be found used in brand new condition for at least a few, if not more off of the retail price. Also, before you decide you want to buy a book, go to your local library and check it out first. I know many time I have heard people say “Well, my library does not carry those books.” Somewhere out there, there is a library that does and libraries are for the people. They want to know what you want. Request books, get your friends to request books and believe me, the library will start buying them.
We moved to a small town with a small library and had a hard time finding the books we wanted. We were limited to 3 Interlibrary loans for each family member and we usually had them all used up every time. When I moved from there, they started when we requested a book, buying it more times than not, and then they started buying books on the similar subjects because they saw how much those books got checked out! I worked at the library for a couple of years and learned how it worked and was surprised how many people just give up and do not use libraries because they think they do not have the books they want. The library here knows me by name as I request so many books!
Half.com and Ebay are great places to buy books cheaper and more often than not it is another homeschool mom you are ordering from, selling it after she read it, depending on the books you wanted.
I sit down with the catalog and go through and search for the books I want to read first at the library. Then if it is somethng I badly need to read then I search to buy.