Although there are a lot of misses, and annoyances, and can be a bit hoaky and juvenile at times...

Wade through the Warhammer 40k books, starting with the Horus Heresy series. Which in summary is the prelude to the 40k series as a whole. The Horus Heresy saga lead into the Siege of Terra books. Of which there are 54 books, and 9 books (more to come) respectively.

There is a lot to read. And a lot of perspectives, motivations, and plotlines that all collapse into a pretty epic story of what essentially is a sci-fi civil war between the forces of "good" and "evil".

And once you've read those (which is already an incredibly challenging task), you've basically completed the introduction to a universe that currently has over 460 books, telling all sorts of stories in the backdrop the Horus Heresy created.

It's pretty wild, and it's got countless great moments and excellent sci-fi.