I'm actually going to go crazy go stupid over how much I loved this book. It's slow, yeah, but Jackson uses her unique and considerable talent to build dread, just as in We Have Always Lived: the prose is gorgeous and lush, but the real star here is her character development. Eleanor is a powerful protagonist, dynamic despite her meekness and not at all what I expected, but Theo and Luke play excellent foils both to her and each other. The trio of assistants make this book more than worth the price of entry. 

Quick aside: having seen both The Haunting (movie) and The Haunting of Hill House (show), I can confidently say that the original book is my favorite when set next to its adaptations. They're all different (and all very good), but the book just does it for me.