Usability 
Here's a new mobile site that helps you find the lowest price for any book. It's a mobile friendly version of bestbookbuys.com. Visit m.bestbookbuys.com and enter the 10 or 13 digit ISBN code found on the back cover of most books and the site will return a list of retailers and prices.
BestBookBuys gets several things right that other mobile price comparison sites seem to stumble with. Prices are prominently displayed in the initial results listing, not buried several clicks deep and include the all important shipping cost. Results are listed by price from lowest to highest with separate listings for new and used books.
BestBookBuys doesn't sell books directly but does link to the sellers' sites. Except for Amazon, most online booksellers don't have mobile friendly order pages. BestBookBuys warns you that the seller's site may not work on your phone and provides the option to email the URL to yourself.
The mobile site is ideal for college students who can visit their overpriced campus bookstore to retrieve the ISBNs of the books they need and enter them on m.bestbookbuys.com. The site claims savings of up to 95% are possible compared with on-campus prices. I tried a few books and found more modest savings of 5 to 30% off the list prices of current technical books. The service is not limited to textbooks either, I was able to find everything from best selling novels to used copies of long out of print computer books on BestBookBuys' mobile site.