Samples are required for all books.
Apple Books automatically creates samples for all books, except Read Aloud books and books created with iBooks Author (custom-made samples are required for those).
What the sample consists of depends on how the book is created. Click here to learn more. If you don’t want Apple Books to cut the sample automatically, you can create a custom sample and add it to the right-hand side of the Files pane in iTunes Producer.
If you created the book with iBooks Author, information about creating a custom sample can be found here.