Definition of "This in only a preview".

Why do I get this in only a preview message?

As a Safari user you have the ability to search across the entire Safari Bookshelf library at all times. However, search results are rendered in two different ways depending upon whether a book resides in the Bookshelf or not. For every book that resides in the Bookshelf you can see every word of that book from cover to cover. For all other books, you can browse or search in preview mode (This is only a preview). Previews consist of the first few paragraphs of text. To request that a book be added, you will need to contact the library directly.