A Blunt Instrument by Georgette Heyer: Who would kill the perfect gentleman? When Ernest Fletcher is found bludgeoned to death in his study, everyone is shocked and mystified. Ernest was well liked and respected, so who would have a motive for killing him? Superintendent Hannasyde, with consummate skill, uncovers one dirty little secret after another, and with them, a host of people who all have reasons for wanting Fletcher dead. Then, a second murder is committed, giving a grotesque twist to a very unusual case, and Hannasyde realizes he's up against a killer on a mission....
Penhallow by Georgette Heyer
Footsteps in the Dark by Georgette Heyer
(I'm on quite a little Heyer reading jag....)
A Foreign Affair by Caro Peacock
Death in Hellfire by Deryn Lake
Behold, a Mystery! by Joan Smith (which...if it takes long enough to get there, could count towards the Christmas Challenge, it's set in Regency England at Christmas time)