Rejected Choices for the Fifth Book

The last choice is a tough one. An atlas might be useful, though a lot of that information would be available in the encyclopedia. A textbook on optics would give them help in making microscopes and telescopes. A book on art might enrich their civilization (just knowledge of perspective alone would be an important advance). A text on trigonometry would be very helpful. Likewise a general textbook on science, or a textbook specifically on chemistry would give a civilization an incredible boost. Perhaps something on musical theory? All good choices, but none of these are my fifth and final choice.