The Twisted Ones by T. Kingfisher
This is a story about a woman and her dog sent to clean out her estranged, and now deceased, grandmother's house. While doing so, she comes across her step-grandfather's manuscript, his attempt to recreate a book he once read and is now missing. Supposedly, the man had come across a non-existent hill that led to strange rock carvings and to something even worse beyond. With huge references to Machen's The White People, Kingfisher spins a bizarre tale that fills you with dread at every turn.