My sister just got into horror and asked me the same thing last month. I steered her toward 'The Haunting of Hill House' by Shirley Jackson. It's unsettling in a quiet way, builds dread without gallons of gore, and the prose is just stunning. A classic for a reason.
For something more modern and punchy, I'd suggest 'Mexican Gothic' by Silvia Moreno-Garcia. It's got a great atmosphere, a determined heroine, and a mystery that unravels in a really satisfying, creepy way. It feels like a gateway into both gothic and body horror without being too overwhelming for someone new to the genre. That or 'The Troop' by Nick Cutter if you want to jump straight into the gross-out deep end, but maybe don't start there.