There are 4 books in the Eisenhorn series. The newest book is The Magos which came out in 2018.

1
Xenos

Book cover of Xenos
The Inquisition moves amongst mankind like an avenging shadow, striking down the enemies of humanity with uncompromising ruthlessness. When he finally corners an old foe, Inquisitor Gregor Eisenhorn is drawn into a sinister conspiracy. As events unfold and he gathers allies - and enemies - Eisenhorn faces a vast interstellar…
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2
Malleus

Book cover of Malleus
A century after his recovery of the alien Necroteuch, Gregor Eisenhorn is one of the Imperial Inquisition's most celebrated agents. But when a face from his past returns to haunt him, and he is implicated in a great tragedy that devastates the world of Thracian Primaris, Eisenhorn's universe crumbles around…

3
Hereticus

Book cover of Hereticus
Hunted by his former allies as a radical and enemy of the Imperium, Inquisitor Gregor Eisenhorn must fight to prove that he remains loyal as he tracks down a dangerous heretic whom the Inquisition believes dead - the dread former Inquisitor Quixos. As he grows more desperate for victory, Eisenhorn…

4
The Magos

Book cover of The Magos
Inquisitor Eisenhorn returns in a stunning new novel that pits him against his oldest foe, forcing him to finally confront the growing darkness within his own soul.

Inquisitor Gregor Eisenhorn has spent his life stalking the darkest and most dangerous corners of the galaxy in pursuit of heresy and Chaos,…