The Unknown Quantity — Mignon G. Eberhart

FATAL DECEPTION

It was a bizarre scheme--and one Sarah should never have agreed to in the first place. Her wealthy oil baron husband wanted to secretly go abroad while a double, James Dixon, took his place at their country estate. Of course, James looked so much like Arthur Travers, it was frightening. And Arthur did insist over and over how important this was to him...How would she refuse?

Once begun, the masquerade turns into a nightmare. A mangled body is discovered floating in the lake by the estate, a secret flame of Arthur's shows up with ominous threats, and thugs harass them in the dead of night. But soon a new crime places Sarah in an even greater danger--chief suspect in the murder of the real Arthur Travers.