Dan Abnett is an excellent sci-fi writer if you like a lot of action. Check out any of his Black Library stuff (its Games workshop affiliated, but don’t let that put you off).
If you can find it the Eisenhorn trilogy (out of print but can get ebook / 2nd hand) or Gaunt’s Ghosts series are great.
I’m presently reading another of his (non Black Library) books, Triumff which I am enjoying.
If you want to get into a long, complex storyline, try the Horus Heresy series of novels. There are various authors, some of the books are fantastic, some OK, but worth it if you are in it for the long haul.