Books for upper-intermediate learners?

I’ve been tested at an upper-intermediate English level (between B2 and C1) and I’m looking for books that are relatively easy to read but still a bit challenging. I want to improve my vocabulary and fluency. I’d prefer stories with very little to no romance (brief mentions are fine as long as it doesn’t turn into a subplot).

I especially enjoy suspense, horror (not gory), science fiction, and fantasy, but I’m open to other genres as long as they don’t involve romance or aren’t very relationship-heavy. Classics are also welcome.

I’d also prefer books that I can find online for free, as I can’t buy books at the moment.

Thank you in advance 🙂