That Affair Next Door introduces Amelia Butterworth, an affluent New York spinster who insists she is not inquisitive even as she watches an apparently empty neighboring house at midnight. A man and veiled woman enter; hours later a woman's body is found beneath a fallen cabinet, her face destroyed and a hatpin driven into her brain. Amelia's observations bring her into direct competition with veteran detective Ebenezer Gryce.
The novel's central achievement is its layered identity puzzle. Clothing, hands, scars, keys, purchases, rings, a stopped clock, and two women of similar build all create plausible but unstable identifications. Howard Van Burnam appears to recognize the victim as his estranged wife Louise, while Franklin Van Burnam is later arrested on a theory involving a stolen letter. Amelia instead follows domestic evidence—the position of a pincushion, alterations to clothing, yarn, laundry work, and the behavior of the mysterious Ruth Oliver.
Green allows Amelia to be comic without making her foolish. Her vanity and certainty can mislead her, but her knowledge of clothing and household habits detects facts the police undervalue. The final confession is long and melodramatic, and the era's language about class, ethnicity, gender, and mental illness is dated. Still, the book is a major early example of a female amateur claiming authority through forms of attention dismissed as merely domestic.
