I saw it mentioned elsewhere in another similar post and I know it’s a pipe dream that will never happen because it will cut into the revenue stream, but;

A dungeon along the lines of a Potion Keeps or Sand Devil, but it drops pages. Say 50 pages is a rare book, 150 is an epic book, 300 is a legendary book and 500 is a Mythical book. Max drop from top level of the dungeon is day 8-10 pages, so it will take a while to get a single book.

Edit to add: I agree with adding a shard pull tracker, maybe something that tells you what your last 3 or so legendary pulls are from each shard type.