The mother-and-son team who write under the pen name Charles Todd have their 20th book in their popular Inspector Ian Rutledge series, “The Gate Keeper.” (William Morrow, 306 pages)

Ian Rutledge, the shell-shocked veteran of the First World War turned Scotland Yard detective, has attended the wedding of his beloved sister, Frances.

Though happy for her, Rutledge is unsettled by the marriage of his only sister and finds himself wanting to take a long drive through the countryside after the wedding festivities.

Near the tiny village of Wolfpit, he comes across a young woman bending over the body of a man in the middle of the road.

The man and woman had been driving home from a party when they stopped to help a man who had flagged them down. But instead of accepting their help, the man shot and killed the victim, Stephen Wentworth.

Rutledge suspects the task of finding a murderer will be far too difficult for the local police and so he takes over the investigation.

At first, he suspects the solution to the crime may be linked to Wentworth’s abrupt breaking off of his engagement to a young woman.

But when another man is killed in a similar fashion to Stephen Wentworth, he looks for the connection between the two men and finds the only clue is an old book on heirloom apples.

Though the book was a valuable collectible, it could hardly be worth the murder of two men. Rutledge must then find the critical clue within this tome of the “poisonous fruit” before a daring killer strikes again.

For those who like historical period mysteries, Charles Todd’s Ian Rutledge series is a fascinating and poignant glimpse of England in the days following the horrors of the First World War and the Spanish influenza pandemic.