Book spine refers to the space between the front cover and the back cover. For a better looking book spine, it is recommended that your page count is more than 48 pages. Still, if your page count is limited to 28 or 32 pages, you can always get good results with the book spine by choosing 200gsm matte art paper or 200gsm gloss art paper for your interior pages. Ideally, the book spine must have a thickness of about 3mm.
The book spine also includes the book title, author name and publisher name. But, in case the book spine space is quite less, it is not recommended to use this space to print any information.
As far as perfect binding and hardcover binding is concerned, the book spine for both will have certain differences. So, where the thickness of perfect binding book spine will depend on the book interior, the thickness of hardcover binding book spine will also depend on the two sheets of boards apart from the book interior.