You have to find the single path across the letter tiles that doesn’t kill you. It is a three-word phrase. By this stage of the game unlocking the chests has revealed two of the words. Those two words are in there and connect to the final word. You need to guess the final word to make it across and find the last books to finish the game. You would also have needed to have found the book that places a bridge to the safe start of the puzzle.