Can be favourite horror author or any genre, really. My all time favourite is Stephen King, and my favourite book by him is The Shining.
As for someone more modern/active, I really enjoy Gillian Flynn's books. She wrote Sharp Objects, Dark Places, and her most recent, and most famous novel, Gone Girl. She's sorta Crime/Thriller, but Sharp Objects is one of the most unsettling books you'll probably ever read.
So yeah, who is your favourite author, and don't be shy about some additional details!