The one I use is Luzme It queries several stores including Kobo, Amazon, B&N and in several countries.
格式不顯示，但亞馬遜網站提供Kindle格式和其他至少ePub - 我會對任何例外感興趣
Amazon only enhanced search engines (I'll group these together since they don't quite fit the question, but are still worth a mention).
Jungle-Search is an interesting search engine. It's not limited to ebooks and is Amazon only (though you can choose from various Amazon stores, such as CAN/US/UK/GER/FRA). The purpose of this search engine is to allow you to more easily search by discount rates and price ranges.
Here is the how-to section (for US since most of our users are US; you can easily switch to another country at the top).
eReaderIQ allows you to search amazon, track books and add them to a watch list (being notified of price drops for books, authors etc.). It also has a history of pricing for ebooks. This is available for US/CAN/UK Amazon sites.