Larian Studios Forums Divinity: Original Sin 2 - The Modding Dungeon Divinity - Original Sin 2 - Modding General Probable workaround to add abilities?

You could simply let them keep the book. When they attempt to use the book (which would have infinite uses), it would check the number of unassigned skill points, as long as they have one unassigned it would reduce them by 1, then give the item. If they don't have any unassigned, nothing would happen.