Warlock Holmes, book 4: The Sign of Nine
G. S. Denning2019

Warlock Holmes may have demons in his head, but now Dr. John Watson has a mummy in his bloodstream. Specifically that of the sorcerer Xantharaxes, who when shredded and dissolved in an 8% solution, results in some extremely odd but useful prophetic dreams. There's also the small matter of Watson falling for yet another damsel-du-jour, and Warlock deciding that his companion needs some domestic bliss...