PoE 3.21: You can no longer create Books of Regression. Existing unused Books of Regression will be destroyed when 3.21 is deployed.
Book of Regression is an item that can be used to reduce the player character’s level by one. It cannot be traded or placed in the stash. The character must have at least one unallocated passive skill point in order to use a Book of Regression.
Books of Regression were removed because their intended purpose (related to PvP characters) is no longer relevant. The way they were being used in Heist was not intended.