One book in 1687 explained why apples fall and why planets don't

Newton's Principia Mathematica showed that the same force pulling an apple to the ground also holds the Moon in orbit and governs every planet's path — one law, one equation, covering motion on Earth and in the heavens alike. Newton told at least two acquaintances late in life that watching an apple fall had set him thinking about gravity, though the famous 'conked on the head' version came later.

— Isaac Newton, Philosophiæ Naturalis Principia Mathematica — 1687