• Insert: put current page number into nbt data
• Remove (at the bookmarked page): erase bookmark data
• Open a written book with bookmark, then it'll jump to the bookmarked page
• Bookmark data won't be copied
• Supports lecterns

This is a planned feature. And my main reason for it is to fix lectern comparator output, which is broken right now.

Lectern comparator output is fixed in 1.1.0 by clicking on page number.
But I might add bookmarks for books in hand in the future.