The Sun may be a mass of incandescent gas, but in the plasmatic reaches of its solar winds spins another seemingly glowing (but relatively minute) orb. The last of the five naked-eye planets discovered in ancient times, Mercury has long been an elusive, enigmatic world. As seen from the Earth, it never emerges far from the Sun, and astronomers in the telescopic era found it challenging to work out basic data such as its rotation period, the inclination of its axis, and whether or not it possessed an atmosphere. In this fully up-to-date and beautifully illustrated account, William Sheehan describes the growth of our knowledge of planet Mercury. From the puzzles it posed for early astronomers to radar studies in the 1960s, and from the first spacecraft fly-bys by the Mariner 10 probe in the 1970s to the latest images from the Mercury Surface, Space Environment, Geochemistry, and Ranging (MESSENGER) orbital mission between 2011 and 2015, Mercury has slowly been brought into clear focus. But although we have now mapped its surface in exquisite detail, revealing strange features like volcanic plains and water-ice deposits in craters near the poles, mysteries remain—such as why its core has the highest iron content of any body of the Solar System. Rather than growing duller on closer acquaintance, this most mercurial of planets continues to fascinate us, offering important clues to scientists as they seek to better understand the origin and evolution of the Earth.