The Watcher In The Shadows by Carlos Ruiz Zafon

In a small coastal town in Normandy during the 1930s, a young girl named Irene Sauvelle and her family move to a mysterious mansion called Cravenmoore, where her mother has been hired as a housekeeper for the reclusive inventor, Lazarus Jann. As Irene befriends a local boy named Ismael, they uncover dark secrets surrounding the mansion and its enigmatic owner. Supernatural forces and a malevolent presence known as the Watcher in the Shadows threaten their lives, leading Irene and Ismael on a suspenseful journey to unravel the mysteries of Cravenmoore and confront the sinister entity lurking within.