Ten Second Staircase

It's a crime tailor-made for the Peculiar Crimes Unit: a controversial artist is murdered and displayed as part of her own outrageous installation. No suspects, no motive, no evidence - it's business as usual for the Unit's cantankerous founding partners, Arthur Bryant and John May. But this time they have an eyewitness. According to twelve-year-old Luke Tripp, the killer was a cape-clad highwayman atop a black stallion.
As implausible as the boy's story sounds, Bryant and May take it seriously when “The Highwayman” is spotted again, striking a dramatic pose at the scene of his next outlandish murder. Whatever the killer's real identity, he seems intent on killing off a string of minor celebrities while becoming one himself.