'Horrid' by Katrina Leno is great if you're looking for horror in the thematic sense rather than a straight up slasher. It's horror and mystery, there are quite a few plot twists, and it's the type of book that builds up the horror through the atmosphere rather than gore. The author has a skill for describing scenery without going into too much detail, it's so easy to imagine the scenes, and there are some gorgeous quotes in the book as well.
Another good one is 'Clown in a cornfield' by Adam Cesare. This one has more gore, it's essentially a slasher, and it has that sort of coming of age vibe going on which blends really nicely with the horror. This one does get a bit intense so if you're not good with gore getting described in detail this isn't the one. This book also has a great plot twist, and it's an overall really fun read imo.
Lastly for a more surrealistic, unreliable narrator type of situation, I'd recommend 'And Then I woke Up' by Malcolm Devlin. Zombies, mental health, politics, and confusion are the words I'd use to describe this book. It can be a hit or miss, as it goes very heavy into the surrealism and there are times when the plot gets a bit confusing, but that's the point of the book itself. If you like short reads which leave you with food for thought, this could be an enjoyable read.