Depends what you’re looking for, but we’ve used https://wisepops.com successfully on a number of sites. Good flexibility, plus, easy integration; don’t need a developer involved to get it up and running, and can do some powerful campaigns. Source: about 1 year ago

My first stop is always bookfinder. Doesn't cross-reference every site but it does a good chunk of them. Source: 12 months ago

If an era (say civil war) go to http://bookfinder.com. Now on the front page click Advanced Search. You can select year of publication. So for civil war I would plug in up to 1900. Source: about 1 year ago

First you need to learn to grade the book, as value is related directly to condition. Then you can look the book up on bookfinder.com and see what it's worth. But... there's a caveat. There's lots of "script sellers" out there, sellers that don't actually own the book - but which scrape the data, markup the price and offer it for sale, planning to buy it when you do. There's also many utter idiots who will... Source: about 1 year ago

If you NEED to purchase a book, use bookfinder.com to find the cheapest books. Always verify the ISBN to make sure you get the correct one edition. Source: about 1 year ago

You should also check bookfinder.com. Source: about 1 year ago