The Others of Edenwell by Verity M. Holloway

Norfolk, 1917. Unable to join the army due to a heart condition, Freddie lives and works with his father in the grounds of the Edenwell Hydropathic, a wellness retreat in the Norfolk brecks. Preferring the company of birds - who talk to him as one of their own - over the eccentric characters who live in the spa, bathing in its healing waters, Freddie overhears their premonitions of murder.

Eustace Moncrieff is a troublemaker, desperate to go to war and leave behind his wealthy family. Shipped to Edenwell by his mother to keep him safe from the horrors of the trenches, he strikes up a friendship with Freddie at the behest of Doctor Chalice, the American owner of the Hydropathic.

As the two friends grow closer and grapple with their demons, they discover a body, and something terrifying stalking the woods. The dark halls of the spa are breached, haunted by the woodland beast, and the boys soon realise that they may be the only things standing between this monster and the whole of Edenwell.