The Vanishing Diary

The Vanishing Diary (1961) by John Rhode (Miles Burton) is the last installment in the Dr. Lancelot Priestley series, featuring Superintendent Jimmy Waghorn. The plot centers on a family dispute over a historic diary, locked in a double-sealed box, which vanishes, followed by the murder of a family member. 

Test pilot Freddie Hapton becomes involved with the feuding Greystoke family in Alderscar, North East England, after his plane crashes. The family, split into two factions, holds keys to a locked box containing a precious diary. When the box vanishes and Patricia, a family member, is murdered, Scotland Yard's Superintendent Jimmy Waghorn is called in.