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Fireplace-puzzle in parents bedroom
You find an incomplete "chess-board" in the fireplace-elevator, which will take you down to the two books:
Numbering the fields on the incomplete "chess-board" starting at the bottom far left corner, left to right = 1 to 8, then next row 9 to 16 etc.... ending up with No. 48 in the upper far right corner field:
Try pressing: 9-34-6-16-30-19-20