A light but pleasant time-slip story.
Jessamy is staying in a large old house for the holidays. Exploring at night, she slips into a cupboard, and when she emerges, it's 1914 and she has taken the place of the housekeeper's niece. She slips into this other Jessamy's place, helping her aunt and the other servants, and playing with the children of the house. And then comes war, and the eldest son runs away to be a soldier, and it seems he's taken his grandfather's treasured Book of Hours...
The plot-line is fairly predictable, but Jessamy is an engaging protagonist, and the secondary characters are well-drawn and believable. And unlike some similar tales, the big house here is no upper-class mansion, but the somewhat vulgar home of a patent medicine millionaire ("good solid middle class", as Miss Brindle says), which makes a refreshing change. And the 1914 and 1966 characters link up in neat but not entirely expected ways, which is also nice.