Deathworld (1960-1968) by Harry Harrison

Wikipedia wrote:Deathworld is the name of a series of science fiction novels by Harry Harrison including the books Deathworld (first published 1960, serialized in Astounding Science Fiction), Deathworld 2 (1964, initially titled The Ethical Engineer and serialized in Analog) and Deathworld 3 (1968, serialized in Analog as The Horse Barbarians), plus the short story "The Mothballed Spaceship" (1973, written as part of a tribute to John W. Campbell). The central hero is a gambler who becomes involved with colonists of an extremely hostile planet. There are several hints that the novels take place in the same universe as Harrison's The Stainless Steel Rat series; however, other hints suggest a similar universe but not exactly the same one.

Wikipedia wrote:Deathworld centers on Jason dinAlt, originally from an agrarian planet Porgorstorsaand, a professional gambler who uses his erratic psionic abilities to tip the odds in his favor. While on a gambling trip to the casino on the planet Cassylia, he is challenged by a man named Kerk Pyrrus (who turns out to be the ambassador from the planet Pyrrus) to turn a large amount of money into an immense sum by gambling at a government-run casino. He succeeds and survives the planetary government's desperate efforts to steal back the money. In a fit of ennui, he decides to accompany Kerk to his home, despite being warned that it is the deadliest world ever colonized by humans.

Pyrrus has a gravity of 2 g; its 42° axial tilt creates severe weather; it is very active tectonically, with frequent earthquakes and volcanic eruptions; and it possesses two moons, Samos and Bessos, comparable in size to Earth's in separate orbits, each creating its own tides that sometimes combine into rises and falls of up to 30 meters, that have been known to flood active volcanoes and create even more violent weather. Finally, there have been numerous supernovae in its vicinity, meaning that planets in the area are rich in radioactive ores. Pyrrus is the only habitable planet in the area, and the only one from which efforts to mine the area's ores can be sustained, making it a world of incredible potential value. It also means the planet's environment contains high levels of radiation.

Everything on the planet is predatory, and capable of killing an unwary human instantly. All large animals are strong enough to destroy small vehicles. All small animals have neurotoxic venom. All plants are carnivorous, even if only by default—their victims fertilize their soil. All microorganisms consume insufficiently protected tissue as quickly as acids. On top of this, all the aforementioned life evolves so quickly that even Kerk and his Pyrran crew have to be retrained upon their return from Cassylia in order to survive.

Because of this harsh environment, the settlers are engaged in a ceaseless struggle to survive, which—despite generations of acclimation and a training regime harsher than that of ancient Spartans—they are losing. Their numbers are less than when the planet was first colonized, and they are restricted to a single settlement. The world's very name is a reference to Pyrrhic victory, a success that comes at devastating cost to the victor.