Best Book Deal says it searches 100 sites, though it doesn't list them. Clearly it searches a lot, including the major names, though a lot are new book sites. Searches only one field at a time. Results show shipping costs and, unusually, condition. Seemed to miss a lot, particularly older material. Link: www.bestbookdeal.com

AAABookSearch says it searches over 50 sites, but only lists the largest, such as Amazon, Abe, Alibris, Half, B&N, and "more." Much of what shows up is new. Only searches one field at a time. Displays shipping costs and condition. Missed a lot of listings. Link: www/aaabooksearch.com

BookButler is a German and newer search site that looks a bit more original than most. It searches 101 sites, with a greater focus on European sites than most, while still hitting the American majors. Advanced search allows you to specify dates and publishers. You also pick by country and choose your currency. Includes shipping and condition in results. A nice step up over most lower tier sites, but sadly only found more recent material. Link: us.bookbutler.com

Isbn.nu searches 14 mostly major sites, including eBay, for both used and new books. It apparently generates more traffic than most of the lower tier sites, although I found it somewhat confusing. Searches up to three fields at once. Displays shipping time but not cost. Did not list older books, but did provide link to Abe for possible matches it did not list, which included antiquarian material. Link: www.isbn.nu

Booksprice does not say what sites it searches, but a test run indicates it hits most majors and plenty of others as well. It offers an advanced search including publisher but not date. Results include shipping and review ratings for the sellers (they like Half but don't much care for A1). Missed many listings. Link: www.booksprice.com

Compricer is a Swedish site that doesn't say who it searches, but most results come from Abe, Amazon, Alibris and England's Blackwell's, with a few others thrown in. Searches only one field at a time. Displays shipping cost. Missed the older material. Suggested trying Abe but did not do the search as did Isbn.nu. Link: books.comprice.com

BookHQ states it searches 24 sites without listing them. Amazon, Alibris, Abe, Half, B&N, Chapters and Blackwell's are among them. Searches one field at a time. Displays only seller and price. Misses a whole lot. A notice says the site is for sale in case you would like to join the crowds in this business. They are looking for high six figures. I wish them luck. Link: www.bookhq.com