Hint!

Try to visualise the pages in a book.

Mentally count through a few numbered pages. If that doesn't help, do this with a couple of real books.

Solution

The book's pages are numbered so that even pages are on the left. That means that 17 and 18 are opposite sides of the same leaf. It's impossible to put anything between those two pages (short of cleaving the sheet in half!). Most books seem to be arranged this way. Have you found any that are different?