The Return of Moriarty (1974) by John Gardner
Good Reads meta-data is 304 pages, rated 3.86 by 982 litizens.
Genre: Sherlock.
DNA: Victorian Britain.
Verdict: Tony needs counselling.
Tagline: Period detail galore.
Tony Moriarty has come back from the dead, making a pact with Sherlock who looms on the horizon but never appears in these pages. Reclaiming the reins of his criminal organisation he dispenses justice, grants favours, invests in heists, and plans his own master stroke. He is one hard working Don. No wonder he needs counselling.
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The telling is replete with the slang of London lowlife of the time and place. The text is accompanied by footnotes that relate incidents to the Holmes canon, which sometimes offer supplementary fictional text to enrich the soup. One in series starring the professor.