Only 20 percent of a book's content normally is available for viewing in a given month on Google Book Search. However, publishers who are interested in publishing advertising-supported books can set the viewable percentage as high as they wish, all the way to 100 percent viewable. When publishers choose this Ad-Supported Access model, Google optimizes its ads for the book, trying to maximize revenue for the publisher. One publisher of travel books has enrolled all its titles in Ad-Supported Access on the theory that it won't hurt sales, while providing more chances that buyers will notice the books and decide to buy one.
Why would someone pay money for a book they can read for free on their computer monitor? A good percentage of travelers who are actually going on a trip to Italy, for example, will prefer to carry a pocket-sized travel book, and will likely buy one on impulse if they happen to see it. At least that's the theory.
Could it ever be possible for Internet publishers to earn more in advertising revenue from online viewership than in selling hard copies of their books? It's an intriguing idea, but it's unlikely we'll know the answer anytime soon. Google admits that ad payments to publishers have not been particularly strong.