I recently came across booksprice.co.uk and thought it could be useful in the run up to Christmas. This is a comparison site where you can check the prices of books, cds, and DVDs. You can search for books by title, author or ISBN and it will check the usual sites (Amazon, bookdepository, Albris, Play, ect). It will also include any shipping costs which is really useful.
One thing I would point out is be careful if you are searching for a book by ISBN. Hardback and paperback editions of a book have different ISBN number. So if you are searching for a hardback edition remember you might be able to get the paperback edition for less (I made this exact mistake a few days ago oops!).