Buying a book means buying the paper the book is printed on, the intellectual content (text) is not bought. You are allowed to resell the paper. And you are in fact not buying a license to use a book when you buy a book, you are literally buying the actual physical book.
If you were buying the paper, then two books of the same page count (even if those pages were blank) would be the same price and interchangeable. They are not. Likewise if you were paying for the paper, then a book with no paper, such as an audiobook, would be free. Again it is not. You are buying some form of media that contains the intellectual content.
