Like the Gill Man himself, this novel by J.R. Fearn (under the pseudonym Vargo Statten) is a fascinating relic from the past unearthed for the world to enjoy. It is overall a much better book than the insultingly terrible and frankly pathetic later effort by Walter Harris under the pseudonym "Carl Dreadstone."
Spanish geologist Dr. Carl Maia is undertaking a dig in the Amazon. His foreman Luís comes to tell him that another one of the crew, Tomás, has found a fossilized arm with a webbed hand embedded in a rock wall the men were excavating. Maia enthuses that in life, the apparently half human, half fish creature must've been a powerful and vicious killer. After removing the arm from the ground, Maia decides to take it to show to some colleagues of his at the Brazilian Instituto de Biologia Maritima (hereafter just referred to as "The Institute"). He leaves Luís and Tomás to guard the camp.
At the Institute, Maia meets his old American friends ichthyologist David Reed and Kay Lawrence. We're also introduced to some other scientists when a meeting is convened to discuss the fossil, including the head of the Institute, Dr. Mark Williams (why an American is the head of a Brazilian facility is not explained). Mark has some bumpy history with David and Kay. He got Kay her job at the institute, and apparently there used to be something between them, but there isn't anymore. Now she is (more or less) with David. However, she still feels some obligations towards Mark because of him helping her with her career. Everybody is just so excited about the fossil, and an expedition is immediately undertaken to return to the Amazon and dig up the rest of the skeleton.
Meanwhile, back at Dr. Maia's camp, evil things are afoot. A living version of the fossilized remains Maia's men found emerges from the nearby river and enters the tent. Luís tries to attack it with a thrown lantern, but the creature deflects his blow and promptly slams him down to the ground, crushing his bones. Then it turns and does the same thing with Tomás, before rampaging through the camp, trashing everything, and then returning to the water.
Meanwhile, we find David, Kay, Mark, Dr. Maia and another American (?) scientist from the Institute named Dr. Thompson aboard a boat called the Rita, commanded by salty old sea dog Captain Lucas, headed up the Amazon River. They arrive at Maia's camp to find Luís and Tomás' bodies, but conclude that a jaguar killed them. While they bury them, Kay wanders over by the river, where the "gill-man" from earlier sees her and becomes infatuated, even reaching a webbed hand out to touch her leg - just as David calls her over to tell her they've finished laying Luís and Tomás to rest.
With the burial out of the way, they set to work, but can't find any more fossilized gill-man remains. It's theorized that hundreds of years ago, part of the rock wall fell into the nearby river. Instead of checking that, they immediately leap to the conclusion that the fossils got carried downriver and emptied into a lagoon which Lucas tells them is called the Black Lagoon. He says it's a "paradise," but nobody has ever returned alive to prove it. Everyone decides going to such a place is the greatest idea in the whole wide world, and, hopping aboard the Rita, they head off thattaway. The gill-man, still with a hankerin' for some human woman, begins following the ship....
Fearn's book is a bit on the lean side, but it's faithful to the film and even expands on it in some places, such as adding a scene where David and Mark have to fight a giant man-eating tree underwater (!). The writing style is a little odd since Fearn sometimes changes scenes mid-paragraph, but once you get used to the flow, it's easy to read. In any event it's nice to have a novelization of this movie with all the characters named correctly, and although I find it odd Fearn changed Lucas and Maia's races (in the movie, Maia is Brazilian and Lucas is apparently supposed to be Venezuelan), the fact they and Thompson don't die horrific deaths as they do in Harris' version is definitely nice.
The book recently got reprinted with a lot of added material including a photo section, an introduction discussing the book's origins and novelizations in general (wherein Harris' book is mentioned and rightfully derided), and an afterward discussing Fearn and how he wrote the gill-man, giving him motivation (in particular, he almost never attacks unprovoked, and pretty much everything he does stems from Luís attacking him in the tent - his first encounter with modern humanity didn't go so well). It comes in both a hardcover and paperback version.