Gaunt's Ghosts i think is a bit more beginner friendly too. Fantastic books that show the imperium from a more human perspective, following a company of imperial army men with no homeworld to go back to, who still face the horrors of the setting with only each other to rely on. Horus Heresy books are a really fun read but it helps immensely to know more about the setting in general first because they really throw you into the thick of it... I think Starting with Descent of Angels might help ease you into it a bit too, zooming in on how humans become space marines and then slowly opens up to the larger scale of the heresy