The House of Lurking Death — Agatha Christie

Throwing on an almost convincing French accent, Tommy is determined to act the Great Detective Hanaud to his and Tuppence's latest, lovely client. Miss Hargreaves has recently received a box of chocolates from nobody knows who, and, due to her dislike of chocolates, was the only one to not fall afoul of the arsenic-spiked treats. But, Miss Hargreaves is not the first recipient of such a gift; three other large country houses have received arsenic-laced chocolates. Miss Hargreaves is holding something back, and Tommy and Tuppence must take up residence at her house to discover the true culprit. But will they find out the truth in time?