A Soul of Steel by Carole Nelson Douglas, 395 pages

Alive and well despite the widely published accounts of her death,  diva Irene Adler and her dashing husband Godfrey Norton are taking coffee with their friend Nell Huxleigh in a Parisian sidewalk café when a stranger dressed in Oriental garb falls at their feet.  It is not Irene's beauty that has felled him, but a dose of poison--and even more surprisingly, the friends learn as he recovers that he is an Englishman!  The mysterious young man informs them that he is seeking a Dr. Watson who tended his wounds at the disastrous battle of Maiwand, a man whose life he says is now in mortal danger.
 
The hunt is on and their search will lead them to a command performance for the Empress of all the Russias and to Sarah Bernhardt, into a channel steamer and at last to the doorstep of  221 Baker Street, where they will discover deadly secrets both past and present.