Waif of the River (Jasper Shrig Mysteries #8)
Jeffery Farnol

mysterious young lady is rescued from the Thames. She then disappears, and a new young lady meets and makes (platonic?) relations with the hero. Great adventures with real peril ensue, the hero falls out with everyone important to him, and dastardly baddies with (of course) foreign overtones do their damnedest.