The mysteries I most enjoy are ones with science underpinnings (forensic science, archaeology, anthropology) and “cozies”. So among my favorite authors are Kathy Reichs and Aaron Elkins (both forensic anthropology authors), Margaret Coel and Dana Stabenow (Native American protagonists), and others. I liked the early Patricia Cornwell books, but eventually I found them too dire, and stopped reading her. Same thing with Jonathan Kellerman. (But for readers who like their suspense and a good fictional thrill, they are both excellent writers!)
In fantasy (and some “hard core” sci-fi, as well) I love interesting, unfamiliar scenarios with relatable characters and novel plots. I’m a huge Harry Potter fan, I enjoyed “Miss Peregrine’s Home for Peculiar Children” (by Ransom Riggs) and “The Night Circus: A Novel” (by Erin Morgenstern). Most good fantasy has some darkness that can be unhappy to read, but most of the time the “good guys” win! And it is intriguing to me to see how the imaginations of outstanding fantasy writers work — worlds of wonder I never could have come up with!
Non-fiction I read for interest, education, and self-improvement, so that category might include works on healthy eating and living, financial management, crafts, and various aspects of science. I am a gardener, so I like plant books and things like “All New Square Foot Gardening” (by Mel Bartholomew). I am a crafter, so I enjoy books on beading and papercrafts, among others. So many books, so little time!!