Minecraft wiki entry for Lectern, associated text: Block entity data
Tags common to all block entities[show]
 Book: The book item, without the slot tag, currently on the lectern, may not exist.
Tags common to all items[show]
 Page: The page the book is currently on, starting from 0, does not exist if there's no book. If higher than the total amount of pages in the book, gets set to -1.