Graham McNeill
Magnus the Red: Master of Prospero
Primarchs (series) part 3

Fighting in the Great Crusade beside his staunchly anti-mystical brother Perturabo, the psychic primarch Magnus and his Legion of knowledge-seekers must walk a fine line between their mission and their own desire to learn forbidden secrets.
The Horus Heresy
novel

Also in:
Primarchs: the Ebooks Volume 1
Primarchs: the Audiobooks Volume 1