Hi Carlito25 - You'll need to override the Treasure Table that's used for the Geo skillbook vendors, or manually add a new treasure table to each of them throughout the various maps. You can view treasure tables in their own section within the Stats editor (in the Shared project).
It can be a little buggy overriding current traders, so you may want to consider just plopping a book (or a crate with the book inside it) into the Fort Joy level; or adding your own trader for it.
You can find more information on modifying treasure tables and adding your own traders here:
https://docs.larian.game/Part_4:_adding_the_skillbook_to_a_vendor
Good luck!
