The Peerless Peer by Philip Jose Farmer

Holmes partners with Tarzan on a quest to catch a nefarious German spy and his weapon of dread in this "glorious" sci-fi mystery from a Hugo Award–winning SFWA Grand Master (Locus)

A night sky aerial engagement with a deadly Fokker aircraft nearly claims three brilliant lives . . . Meanwhile, Baker Street's enigmatic mystery-solver, Sherlock Holmes, forms a historic alliance with Greystoke—the noble savage, peer of the realm, and lord of the jungle—bring down the hellish spy Von Bork!
This edition also contains a brand new afterword by Win Scott Eckert and a bonus preview of Kim Newman's novel, Moriarty: The Hound of the D'Urbervilles.