Which 1596 Kepler book was an early published defense of Copernican astronomy?

The 1596 Kepler book that offered an early published defense of Copernican astronomy was Mysterium Cosmographicum.

Johannes Kepler published the work, whose title means “Cosmographic Mystery,” in Tübingen late in 1596. It defended the Copernican arrangement of the Solar System and tried to explain the spacing of the six then-known planets through the five Platonic solids. Kepler imagined nested geometric solids and spheres representing the planetary orbits.

The book was not a modern scientific defense: its reasoning blended geometry, theology, and a belief that the universe reflected divine mathematical design. Even so, it was an important early attempt to argue that heliocentrism described the physical structure of the cosmos, rather than serving merely as a convenient calculating device.

Kepler sent copies to leading astronomers, including Galileo and Tycho Brahe. Tycho's response helped draw Kepler to Prague, where access to Tycho's observations eventually led to Kepler's laws of planetary motion. The book received an expanded second edition in 1621, while Kepler's later works replaced its speculative geometry with more accurate orbital mathematics.