Here's a selection from the Amazon new and used photography book listings. These books are mostly under $10 (or were when I last looked) a few are under $5 - some are even as low as $0.10! However all of them are heavily discounted from the cover price and all of them with Amazon ratings have 4 to 5 stars. Shipping is $3.49 per book, but even with shiping included they all represent great value and a very significant saving over their cover prices, which range up to $50.
In some cases there may be only one copy available at the listed price so if you see something you want, grab it!
I've personally bought many new and used books via Amazon and I've never had any problems with sellers or the condition of the books I've received. Many of the "used" books are in fact brand new, but being sold off at discount prices.
