The 10-digit number on the back cover of every book. Some people even think that books may be most suitable for online shopping. At least that was what amazon's founder Jeffrey Bezos concluded at the time. Indeed, no matter where you buy around the world, books under the same title from the same publisher with the same ISBN are always the same.
Books are easy to ship too.
Be aware that different stores discount different books. If you search multiple book e-tailers, it is almost certain you can find a good deal which is better than you can get from your local bookstore even after you take the shipping charge accounted. Titles come with most extensive comments and rating by readers, authors or publishers. Excellent customer service. Titles also come with extensive comments and rating.