In general, knitting books are in high demand so it can be really hard to find them cheap, depending on the book of course. I’ve probably gotten the best deals from Amazon Marketplace sellers, of all places. :shrug:
Amazon marketplace is where I’ve had the best luck, too, for books and such. Also, and I don’t remember which podcast I heard this on, but Tuesday Morning apparently has really great deals on knitting things. Supply and quantity vary, of course, but it’s definitely worth looking if you have one in your area!