When, eight years after its original appearance, the novel was finally published in the U.S., the eminent American critic Anthony Boucher lauded it in his column in the New York Times. Noting that Nightingale was an amateur tenor, he described him as an “unusually attractive” character. For Boucher, the book was “fascinating as a stage in the development of an important writer, and a pleasing entertainment in its own right.” Kirkus Reviews also approved the novel, pointing out that the story involved “more pursuit than procedure,” and saying it was “easy to read, fast to follow, with no remission of interest.”

Mary Kelly’s love of music is evident in much of her fiction, including The Christmas Egg. She was an enthusiastic singer, a mezzo-soprano with an extensive knowledge of lieder, capable of singing Schubert’s song cycle from memory. Brett Nightingale’s wife, Christina, was an opera singer, and Mary gained insights into the life and work of professional singers from Monica Sinclair, a member of the Covent Garden Opera Company who was Sir Thomas Beecham’s preferred choice as contralto. Critics who shared Kelly’s devotion to opera, and derived particular pleasure from her work, included two notable commentators usually associated with the classic whodunit rather than the psychological crime novel, namely Boucher and the composer Bruce Montgomery, better known as the detective novelist and Sunday Times reviewer Edmund Crispin.

The Christmas Egg consolidated Kelly’s developing reputation as a quirky, intelligent crime novelist, but she was never interested in following fashion or working to a template. Later, in an excess of modesty that seems typical of her, she would describe the three Nightingale books as “sins of my youth.” She wrote a novel without Nightingale, Take Her up Tenderly, which was rejected and never published. Her next book, which appeared in 1961, was very different, and it heralded a breakthrough in her literary career.

The Spoilt Kill was set in the Staffordshire Potteries, and had a workplace setting as memorable as it was unusual. The protagonist was an enquiry agent called Hedley Nicholson, but he was as unlike, say, Sam Spade or Philip Marlowe as Nightingale was unlike Freeman Wills Crofts’ Inspector French or Ngaio Marsh’s Inspector Roderick Alleyn. The book was critically acclaimed and won the Crime Writers’ Association Gold Dagger for the best crime novel of the year, which was presented to her by Sir Compton Mackenzie. Given that in winning the prize, Kelly’s book edged out John Le Carre’s Call for the Dead, which introduced the now legendary George Smiley, the scale of her achievement is clear. She was promptly elected to membership of the prestigious Detection Club at the age of thirty-four; later, she became the club’s secretary.