There is really no limit to what a book can be or what it can be made from. Books have been made from food, fabric, wood, metal, glass, found objects and anything else you can think of.
They can also be about anything you can think of. Or they can be blank, ready to be filled with wonderful thoughts. Here are some examples of people thinking outside the traditional book parameters. Please click on the images to visit their sites and learn more about their works.
This book from Angela Lorenz entitled Soap Story is printed on soap.
Rosemary Furtak created this work.
Amazing artist books from Brian Dettmer.
Robert The's Whimsical cake book.
I encourage you to Google Artist books and Artist's books in images. You will be amazed at all the creative artistry that there is out there.