The Horus Heresy: Primarchs
It's the start of a brand new series of 18 books focusing on the greatest of heroes, the primarchs themselves. This volume kicks it all off with a tale of Roboute Guilliman at the height of his powers, leading his legion into battle at the apex of the Great Crusade.
The Horus Heresy: Primarchs consists of seventeen works, and the series is set to expand with the upcoming release of one more book. The current recommended reading order for the series is provided below.
Main series Warhammer 40,000: The Horus Heresy