i really enjoy horror/mystery!

Might I recommend 14 by Peter Clines. It should scratch that itch.

The Silent Patient is another good mystery one.

If you can, go to your local public library and have a chat to the librarians. They are a goldmine of quality recommendations.

My favorite recent book is Piranesi, by Susanna Clarke! It's a mystery/fantasy with light horror elements, and thoroughly stole my heart. I also recommend checking Standard Ebooks for high quality, free, public domain books! If you have some money, an eReader can really help you save money in the long run, or you can read on your phone using book apps like KoReader!

How old are you then?

For horror, Stephen King is one of the best. Start with something shorter of his, a short novel, or one of his collections of short stories. His longer novels are great too, but his short stories are where he really shines.

A couple of my favourites are:

1408

The man in the black suit

Pet Cemetery

The raft.

The cycle of the werewolf.

Happy reading!

Edit: for mystery I enjoyed Six Wakes by Muir Lafferty, a SciFi murder mystery on a spaceship. I also enjoyed Agatha Christie's Hercule Poirot stories, they're fairly simple and easy to read.

Don't know if it's your taste, but my most recent favorite is The Dispossessed which is a political sci-fi kind of book about a brilliant mathematician who lives in an anarchist world

Dracula is my go to. There is a daily email you can sign up for that follows the books entries. Jurassic Park is good as well.

You don't really give any information that could be used to personalize this recommendation, but I can tell you that the classics have become classics for a reason. You might not personally like them all, but no book remains being read over centuries if it sucks.

Maybe try The Fisherman by John Langan. https://en.wikipedia.org/wiki/The_Fisherman_(novel)

I usually read sci Fi stuff but that was a good horror mysterious one.