Hints
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
Backup Power Box that Yeesha points out
While in Tomahna at the beginning of the game, do not open the glass cage containing the bugs unless you want Yeesha to get angry with you for ruining her experiment.