How do I share a character or a place from one book with the whole series?

From version 1.9.24, open the Codex of a book in the series, choose "This book", and press "Share with series" on the saved character card or world entry. It moves to the series Codex, so every book of the series is written with it. Anything you typed on that card is saved first. If the series Codex already has a character with the same key, nothing is moved and BookyAI says so: change the key of one of them first. In a series book, "This book" reminds you that its entries belong to that book only, and that Series is where you share them.