The Soprano's Last Song (2014)

Book 2 in the young adult "Sherlock, Lupin & Me" series by Irene Adler.

Irene Adler, Sherlock Holmes, and Arsène Lupin planned to reunite in London, England, for their second adventure together...but Lupin doesn't show up. His father, Théophraste, has been arrested for the kidnapping of a famous opera singer and the murder of a secretary to the great composer, Giuseppe Barzini! The three intrepid detectives start a full investigation to clear Théophraste's name and rescue the missing soprano, Ophelia Merridew, only to discover that the singer is being held captive by criminals in the deadly district of Bethnal Green. Join Irene and her friends in their twist-filled bid to solve the mystery of The Soprano's Last Song.

Title: The Soprano's Last Song
Author: Irene Adler
Year: 2014
Publisher: Stone Arch Books
Purchase: Amazon.com | Amazon.co.uk

The Case of the Revolutionist's Daughter (1983)

Sherlock Holmes meets Karl Marx in an adventure by Lewis S. Feuer published in 1983. I'm currently selling my copy on eBay.

At last, devotees of the adventures of Sherlock Holmes can learn of his most challenging, and, ultimately, his most disappointing investigation. Fortunately, a codicil in the past will and testament of socialist historian and Holmes admirer Professor Harold J. Laski has finally released the one remaining copy of this tale of intrigue from the custodial care of Lewis Feuer.

"The Case of the Revolutionist's Daughter" begins with Holmes getting with Karl Marx and Friedrich Engles. The task? To locate Marx's missing daughter, Eleanor, and to return her safely to her parents home.

Holmes search leads him into the Bohemian socialist intellectual world of London in 1881. With his trusted friend Dr. Watson in tow, Holmes traces Eleanor's trail from the dark archives of the British Museum library to Kelmscott House, a socialist meeting place where Ibsen plays were read aloud. The famous superseleuths me such luminaries as George Bernard Shaw, Beatrice Potter, and William Morris. They confront the ominous atheist Edward Aveling and his villainous cohort, professor Moriarty, who holds the key to the mystery, and to Eleanor Marx's fate––which Holmes argues, may have been determined long ago by her father's philosophical stance and human failings.

Title: The Case of the Revolutionist's Daughter
Author: Lewis S. Feuer
Year: 1983
Publisher: Prometheus Books
Purchase: Amazon.com | Amazon.co.uk | eBay

Death By Gaslight (1982)

Michael Kurland's second Professor Moriarty novel. Once again the paperback sports nice cover art with a Rathbone inspired Holmes.

BLUE BLOOD WAS FLOODING LONDON WITH HORROR

A killer who made Jack the Ripper seem gentle was slitting the throats of the cream of England's aristocracy. Naturally Scotland Yard launched total war against this monster, enlisting the great Sherlock Holmes himself.

Only when this ultimate weapon of the law failed to stem the blue-blodded deaths were they forced to play a last desperate card–Professor James Moriarty, the Napoleon of Crime, who had his own methods of fighting an evil that went beyond mere murder by gaslight to hell-lit depths of diabolical depravity and satanic sin....

Title: Death By Gaslight
Author: Michael Kurland
Year: 1982
Publisher: Signet
Purchase: Amazon.com | Amazon.co.uk