Better yet, it should be the author's or publisher's responsibility to create a preview version of the book. That will allow them to determine both the amount and portions to include. For something like fiction, it may be the first couple of chapters. For a book on history, it may be the introductory material plus the first chapter. Other types of non-fiction may choose representative chapters (e.g. teachers manuals may include a sample lesson, electronics projects books may include a sample project). A new author may include most of the book, leaving a cliffhanger to encourage the reader to buy the remaining few chapters. But samples as they stand today can be useless at times. Even a better automated approach that incorporates will likely fail because the nature of books varies from genre to genre and from author to author.