The site might then compile a list of books by the same authors, or books on related topics, and show you that list. This activity is invisible to you. Unless you have set your preferences so that you will be alerted when a cookie is being stored on your computer, you won't know about it. When you return to a web site, you won't know that a cookie is being read. From your point of view, in the example above, you'd simply visit the online bookstore, and a list of books that might be of interest to you would magically appear.
Cookies are usually harmless.
But some services do use cookies to create a profile of your interests based on the sites you visit and the things you do there. Advertisers on participating sites can then tailor online advertising to your interests and buying habits.