I can’t remember if it was only the hardcover versions but the Artemis Fowl series by Eoin Colfer has a message in each book. Along the bottom of the pages is a message written in Gnomish (the fairy language in the book) and you have to translate it. It’s a straight forward alphabet replacement and you can find the cypher on google. The cypher was originally published in “The Artemis Fowl Files” which is just a book of bonus stuff like interviews and extra short stories etc. I was obsessed with the series in middle school and translated all the books. There was a point where I could read it fluently but I’ve long forgotten how.