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