Which 1619 Kepler book presented his third law of planetary motion?

The 1619 Kepler book that presented his third law of planetary motion was Harmonices Mundi, usually rendered in English as The Harmony of the World.

Johannes Kepler published the book in Linz after years of studying planetary data gathered by Tycho Brahe. Its five books explored geometry, musical harmony, and astronomical relationships, reflecting Kepler’s belief that mathematical order connected the heavens and the physical world.

The third law appears in the work’s final section. It states that the square of a planet’s orbital period is proportional to the cube of its average distance from the Sun. This links a planet’s distance and year length quantitatively.

A common mix-up is with Astronomia nova, Kepler’s 1609 work, which established his first two laws: elliptical orbits and equal areas in equal times. Mysterium Cosmographicum was his earlier 1596 attempt to relate planetary spacing to the five Platonic solids.