The Companion by Sarah Dunnakey

1932 - Twelve-year-old Billy Shaw lives in a palace. Potter's Pleasure Palace - the best entertainment venue this side of the Pennines, complete with dancing and swing-boats and picnickers and a roller-skating rink. Billy's ma runs the tea rooms and Billy himself looks forward to becoming the assistant to Mr Potter when he grows up. Until, that is, Mr Potter arranges for Billy to go to High Hob, the big house at the top of the valley, to be companion to Jasper Harper.

Jasper lives with his mother Edie and his Uncle Charles, brother and sister authors, escaped from London, and some say debt and scandal, in order to write. On his arrival, Billy finds a haphazard household where nothing that's meant is said, and he runs wild with the untamed Jasper, spending all the time they can on the moors trying to catch The Beast. For four years the boys are inseparable, but when Charles and Edie are found dead, ruled a double suicide, Billy has already left the valley to start a new life in London. His time in the Harper household is written out of history.

2015 - Newly-appointed custodian of Ackerdean Mill, formerly the Palace, Anna Sallis begins to sort through the chaotic archives of the Mill, the Palace and the Harper siblings, and finds documents pointing to inconsistencies in the accepted story of Charles and Edie's suicide. Anna becomes curious about what happened to her neighbour Frank's Uncle Billy, absent from the known story. Why did he leave the valley? And what did he know about the events at High Hob?