AddAll is also quite popular. Says "40+" booksellers, though only 22 are listed for used books. They also search four versions of Abe and five of Amazon, along with other major names like Alibris, Choosebooks, Half and some of the smaller sites and larger individual booksellers (Powells, Strand). An advanced-type search without dates is offered. Searches are faster but not always as thorough. They also find old and rare books. Presents all listings together, the way Abe displays results. Link: www.addall.com

Fetchbook is ranked third by Alexa. It searches 126 sites. You would think everything would be covered here, and the list is certainly extensive, including the important but sometimes overlooked site eBay. Still, ILAB and the French Livres-rare-book, searched by the previous sites, are missed by FetchBook. No one gets them all. Only allows one field at a time to be searched, not very convenient for pinpointing titles. Appears to miss pre-ISBN editions. Link: www.fetchbook.info

BookFinder4U is one of two sites that appear to be on the next rung in terms of traffic. Newer to the web than the more established competitors, they promote aggressively and have lined up 128 sites to search, two more than Fetchbook. They hit just about all of the sites, though no one is perfect (Livre-rare-book is missing). They offer an advanced search that doesn't have dates, but is one of the few that includes publisher. They post estimated shipping costs. Has a separate search specifically for rare books. Link: www.bookfinder4u.com

All Bookstores is the other service on the next tier. They search 31 sites, including the big names, Amazon, Abe, Alibris, Barnes and Noble, Biblio, Half, eBay, some smaller ones like ElephantBooks, and some sites with just newer books, such as Overstock and Wal Mart. Searches only one field at a time. They include shipping details such as cost, estimated time, shipping methods, etc. Seemed to miss a lot of material on the sites they search, at least when we tried it. Link: www.allbookstores.com

UsedBookSearch was once among the leaders, but seems to have faded. They search just 11 sites, but since the majors are here, it probably still finds most books. Offers a three-field search. UsedBookSearch displays results in an unusual way. It shows the number of matches for each website. Clicking on the number takes you directly to the results displayed on that site. The disadvantage is that this makes it hard to compare copies and prices from one site to the next. However, they do find all types of books, including rare and antiquarian. Link: usedbooksearch.com.uk

Bibliofind was probably once the largest. It was purchased a few years back by Amazon and now it only searches Amazon's site. In other words, it is pointless. You can just as well go to Amazon. Link: www.bibliofind.com