By Marcia Muller and Bill Prozini

When bicycling through Golden Gate Park ends with the revelation  of threatening letters and a gunshot in a mansion garden, Sabina Carpenter knows this is a case that demands her immediate attention.  The questions her partner John Quincannon has to unravel are easy.  The hard part is the dead courier, murdered in a locked room within a locked room, creating a trail that will take Quincannon through San Francisco’s less savory places and end with a riverboat trip that is anything but a relaxing cruise.