Death and the Princess — Robert Barnard

THE QUEEN WAS NOT AMUSED.
Murder and royalty didn't mix. Princess Helena had only a distant claim to the throne, but when her friends and lovers began turning up dead, Buckingham Palace demanded to know why. The princess appeared to have a wildly wicked streak. It might have been elegant entertainment indeed, squiring Her reckless young Royal Highness through her dutiful official days and licentious nights. But Superintendent Perry Trethowan, Scotland Yard, had precious little time for pleasure if he meant to stop a cold-blooded killer intent on blue-blooded murder and prevent a scandal that could shake the nation.