You know something? This isn't really all that good, no. But if you wish Alan Moore wrote more name-dropping, allusion-filled novels with very little substance, this is right up your alley. Mr. Farmer peppers the book with late-Holmes-era British fighter-hero-guys, most of whom were probably more readily recognizable when the book came out fifty-some years ago. As you know, people aren't allowed to know about things that happened before this morning, so you might be really confused by this book.
Very little actually happens in this book. Mr. Farmer expects you to know a decent amount of Holmes facts, which, again, may have been a more reasonable expectation in the 1970s, but that knowledge neither helps nor hinders your experience of this book. I don't want to overwhelm you with scientific profundity, but over 87% of this book is Old Holmes and Old Watson getting places, and as soon as they get to that place, they have to get to another place. Inexplicably, Old Holmes and Old Watson are jumping out of airplanes, hiking through jungles, swimming across rivers, climbing up mountains, and, frankly, even for a work of fiction comprised of imaginary characters, it's a load of codswallop. Mr. Farmer emphasizes throughout the book Holmes and Watson are very, very old (even older than forty ... like, tremendously old), yet he has them do thing not even the X-Men could reasonably do.
You might suspect from the cover and the title and the author that Holmes and Watson would quickly meet Tarzan (or Mowgli in other versions, because he's less ... offensive?), and the three of them investigate Tarzany mysteries and solve Tarzany problems. You would be completely and totally wrong.
Ancient Holmes and Ancient Watson don't even meet Tarzan until 60% through the adventure (if "adventure" meant "going from place to place, hilariously watching Super Old Holmes vomit because of air travel for pages at a time). Then, when they finally meet Tarzan, Mr. Farmer spends a boggling amount of time trying to convince us this isn't even the "real" Tarzan, but some sort of homicidal imposter. It's very, very confusing.
Tarzan has nothing to do with the covert plot, which is something about the World War 1-era Germans have created the Stereotype Killer (not its actual name, nor is it a socially-helpful destroyer of stereotypes, but a "this will make ethnic groups sad because it ruins the stereotypical thing they love nonstop all the time" sort of awkward thingamabob). Painfully Not Young Holmes flips a coin toward the end of solves the mystery ... I dunno ... it was really confusing, especially the part of Mega-Ancient Holmes coaxing Tarzan/Fake Tarzan into paying him a bajillion pounds (he has a checkbook handy in his leopard skin trousers, or something - I can't remember) for helping him ... stop the ... wait ... fight the .... Actually, I'm not sure what Tarzan is doing in this book. He's in it for less time than you-know-who in The Third Man, except he doesn't really contribute other than killing a lot of World War 1-era Germans, though he sort of rescues Cripplingly Moribund Holmes and Watson from a tribe of More Stereotype Tribesmen. Fortunately, this gives Ancestral Watson the opportunity to kidnap-rescue a woman and trick her into marrying him (I think her name is Anna Nicole, but I could be remembering that incorrectly).
It's hard to remember most of this book correctly, because it's not that memorable. But it does have Not Really Young At All Holmes dressing up as a bee to talk to bees while dressed as a bee, because bees think a six-foot-tall Walking Skeleton Of A Man is a large bee if painted the right colors. So there is that.
You may want to skip this one, unless you are the World's Hugest Sherlock Holmes Completionist in the History of Mankind.