The book will have a small button on the side. For example you have a closed book in your both hands. Is the side of the book the part which is in your hand. I mean the part that pages are joined together by glue?
Sorry, Naamplao - the binding of a book is the entire cover, front and back boards and spine [which is the part where the pages are held together], covered with cloth or paper.
Without considerably more context, I would interpret a button on the side as being on the front or back board for a loop to fasten over.