Make sure you have shut off everything in the Waterfall room before pushing the buttons at the Power Box. Click combinations until both symbols for the Waterfall are green. Then, go back and open the floodgate; watch your head.
There is an incomplete "chessboard" in the fireplace-elevator, which takes you down to the two books. Number the fields on the incomplete "chessboard", starting at the bottom far left corner, going left to right: 1 to 8. Then for the next row: 9 to 16, and continue the pattern with 48 in the top far right corner. Note: Try pressing: 9-34-6-16-30-19-20.
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.