The killer dresses in drag but that detail gets short shrift. Was the killer accustomed to wearing women's clothes? Dannay and Lee missed an opportunity to establish that the killer was a man by not having him get out of the car when he stopped to get gas. Isn't that what men did when they got gas? Get out to stretch their legs? That would have also revealed the killer's footwear. High heels? Establishes history of wearing women's clothes! Flats? What women wears flats with that ensemble?
Also, Ellery doesn't say that it's probable that the killer was a man because they smoked a cigar or a pipe while they waited for their victim. He says that it's a certainty that women don't smoke cigars or pipes. Could have said that the women in this case wouldn't smoke a pipe but says ALL women don't smoke pipes.
Audiobook read by Fred Sullivan, published Blackstone Audio in 2014.