Need an idea for a Sudoku Christmas gift

My father is heavily into Sudoku and has been solving them for decades. I regularly gift him things like Sudoku Calendars but to avoid being too repetitive, don't just want to buy a random one from the store. So since I don't really do Sudoku myself hence coming to you guys for advice: Do you have any suggestion for what could be a cool gift for a Sudoku enthuisast? Could be particularly unique puzzle books are something interesting Sudoku adjacent.

Only limitation is it must be purchasable in central europe.