Obviously, they don't show you all or even most of the book. But much of it is available this way. And depending on if you're visit Amazon.com, or Amazon.ca, or Amazon.co.uk, different amounts of the book may be available, depending on the book. I sometimes read the excerpt to a book on another's country's Amazon site.
Clearly, this sneak peak is designed for shoppers to see if they want to purchase the book. I use it to see if I want to check the book out from the library. And then, perhaps I may want to own a specific book. Most of the time, though, with self-help books, I can get the gist of the information, enough for my tastes, in just the sneak peak. It's just another frugal indulgence of mine.
I mentioned the sneak peak feature to a friend who had ordered a book from Amazon, recently, and this friend was very surprised that this even existed. So, I'm sharing, in case you don't know this either.