The Return of Moriarty [Prof. Moriarty #1] by John E. Gardner

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London is in terror. The streets are filled with dippers, macers and bullies of every description, all collecting "contributions for the Professor." But Holmes saw Professor Moriarty swept over a waterfall in Switzerland! Could it be that Europe's Master Criminal somehow survived, and has returned to battle Holmes again in the greatest crime duel of all time?
It could indeed.