We do not buy a lot of books. We depend heavily on the library and on the internet for entertainment and reference. We do have a lot of children's books, most of which were gifts. (Even with all those books, the kids still want to check out more at the library when we go.) When we do buy books, they are almost always non-fiction or fiction that we will use for school for multiple children.
I find I rarely read fiction anymore. Most of my reading for entertainment happens at blogs. I do take an occasional fiction book from the library, but I don't feel any need to buy them. I rarely read a book twice, so purchased fiction just tends to take up space and gather dust.
If we need a book for school I borrow them from the library first. If we like the book and will need it for an extended period or can use it for another child later, then I will buy it, used if possible. When we started teaching Lydia reading, I borrowed a copy of Teach Your Child to Read in 100 Easy Lessons from the library. I kept renewing it until they wouldn't let me anymore, and then I bought a used copy.
Most of the books you'll find on my bookshelf are ones that I reference frequently. There are books about gardening, wild plants, field guides, Christianity, and history mixed in with a few select fiction. Though you can find almost any information you need online, I sometimes find it just more convenient to have it in print.
If I want a book on a certain subject, I hit the library shelves first. The biggest advantage of using the library first is that you can try before you buy. You can take the book home and see if it is worth keeping around before you spend your money on it. The other advantage I've found is the library has a lot of older titles. Older titles mean I kind find the book used at a much lower cost compared to newer editions. And really how much changes in most nonfiction books? I actually prefer the old fashioned how to's most of the time!
When it is time to buy I look for used. Library book sales are wonderful. You can find some gems for almost nothing, but if you are wanting a specific book that is probably not the best place to go. I go to Amazon and Half.com. Half.com will let you see related eBay auctions also. I have purchased a few books from eBay. These sites allow you to purchase used books for much less than they would be new. The shipping on books is usually pretty reasonable, but be sure to consider that when you are comparing prices.
We love books, but we love using them from the library best of all. Before I buy a book, I want to be sure I will get a lot of use out of it. If I am going to buy it, most likely it will be second hand. Here are a few books that have recently moved from my library list to my buy wish list:
Encycloepdia of Organic Gardening This is an older gardening book, but has lot of information about companion planting, pest control, and general gardening in an easy to use format. It even has a small section on foraging for wild plants, of course that is always a selling point for me!
Back at the Farm: Raising Livestock on a Small Scale Again an older book, but wonderful for beginner homesteaders! This book is a great introduction for those starting to raise animals. There are small sections (4-5) pages on the most common farm animals. They had sections for all the animals we are raising now. The information includes feeding, shelter, profitability, and butchering.
My Indian Boyhood Alright this isn't really a book about frugal topics. It is a book Kellen needed for school, but I am loving it. The author was the son of a Lakota chief born in the 1850's. He talks about growing up in his tribe before they lived on a reservation. It is a history and culture book. That I really enjoy, but the resourcefulness and frugality of these people is inspiring.