It's an authorized Tarzan book! Cool. Farmer doesn't try to mimic the Edgar Rice Burroughs style, so that's refreshing. The story is set between the events of Tarzan the Untamed and Tarzan the Terrible. Tarzan is in the middle of tracking down the Germans who kidnapped Jane, but first he must deal with some minor bothers.

Sasquatch
The minor bother is that someone is hunting him. From far away in America, a wealthy oil baron is orchestrating a plot to capture Tarzan and study him to learn the secret of immortality. (Oh, did we not mention that Tarzan is immortal?) To that end, a safari of white guys and their hired African tribesmen are hunting Tarzan. But there's something else in the jungle too: not man, not ape, but something other. Tarzan calls it a Ben-go-utor, but I call it a Sasquatch. The men who are after Tarzan have imprisoned (or perhaps killed) the sasquatch's mate and are forcing it to help them hunt Tarzan. Eventually Tarzan meets the sasquatch, who is named Rahb and who may be the last of his kind.

Tsathoggua
Tarzan and Rahb team up and head into a remote part of interior Africa where they encounter a strange civilization of men who worship an ancient frog god. Not other that, but there's a crystal from outer space called the Dark Heart of Time, which actually involves time travel or something, and plus there are all sorts of hints about elder gods and whatnot. This really sounds like some H.P. Lovecraft, and it sounds even more so when they find the frog god itself: a squat alien monstrosity that is clearly not from this world. Tarzan kills it.

My verdict
Maybe it's not quite a story Edgar Rice Burroughs would have written, but it's an excellent addition to the Tarzan canon. I approve.