The Husband's Secret by Liane Moriarty

From Goodreads:

At the heart of The Husband’s Secret is a letter that’s not meant to be read

My darling Cecilia, if you’re reading this, then I’ve died...

Imagine that your husband wrote you a letter, to be opened after his death. Imagine, too, that the letter contains his deepest, darkest secret—something with the potential to destroy not just the life you built together, but the lives of others as well. Imagine, then, that you stumble across that letter while your husband is still very much alive. . . .
Cecilia Fitzpatrick has achieved it all—she’s an incredibly successful businesswoman, a pillar of her small community, and a devoted wife and mother. Her life is as orderly and spotless as her home. But that letter is about to change everything, and not just for her: Rachel and Tess barely know Cecilia—or each other—but they too are about to feel the earth-shattering repercussions of her husband’s secret.

Lockwood & Co: The Screaming Staircase

A sinister problem has occurred in London: all sorts of ghosts, haunts and spectres are appearing throughout the city. Only young people have the psychic abilities required to see and eradicate these unnatural foes. In 'The Screaming Staircase', the plucky and talented Lucy Carlyle teams up with Anthony Lockwood, the charismatic leader of Lockwood & Co, a small agency that runs independent of any adult supervision. After an assignment leads to both a grisly discovery and a disastrous end, Lucy, Anthony, and their sarcastic colleague, George, are forced to take part in the perilous investigation of Combe Carey Hall, one of the most haunted houses in England. Will Lockwood & Co. survive the Hall's legendary Screaming Staircase and Red Room to see another day?