The Adventure of the Peerless Peer

The Adventure of the Peerless Peer is a 1974 adventure pastiche novel written by Philip Jose Farmer, writing as Dr. John H. Watson, about the meeting of Sherlock Holmes and Tarzan. This was one of several works Farmer wrote that involved Tarzan.

Plot

The story is presented as a lost manuscript of Watson's, edited by Farmer. On orders from his older brother Mycroft, Sherlock Holmes and Dr. Watson travel to Africa and form an alliance with the titular peer, Lord Greystoke a.k.a. Tarzan, to hunt down the nefarious Von Bork and stop his deadly new weapon.
Also mentioned:

6th Duke of Holdernesse
Sir Henry Merrivale
flight lieutenant John Drummond, adopted aon of current duke, possibly Ace Drummond
The Spider
Handley Page Type O aircraft
The Shadow
Sikorsky Ilya Muromets aircraft
L 9 aircraft
Pemberley, now residence of Duke of Holdernesse
Peter Carey
Kingdom Zu-Vendis
Allan Quatermain
Sir Henry Curtis

Reissues

Due to copyright issues, it was rewritten as "The Adventure of the Three Madmen"—with Mowgli from The Jungle Book replacing Tarzan—in The Grand Adventure collection. It was reissued by Titan Books in 2011 as part of The Further Adventures of Sherlock Holmes series with Tarzan and the original title restored. It has the abbreviated title of The Peerless Peer.