Everything Under by Daisy Johnson
It's been sixteen years since Gretel last saw her mother, half a lifetime to forget her childhood on the canals. But a phone call will soon reunite them, and bring those wild years flooding back: the secret language that Gretel and her mother invented; the strange boy, Marcus, living on the boat that final winter; the creature said to be underwater, swimming ever closer.
In the end there will be nothing for Gretel to do but to wade deeper into their past, where family secrets and aged prophesies will all come tragically alive again. 'As readable as it is dazzling, full of unsettling twists and dark revelations' Observer