I recently purchased a collection of his works in a hardcover three-volume collection, and so am giving my original paperbacks to others to enjoy.
Mission of Gravity - Penguin paperback edition, 1963 printing. Humans hire native aliens to retrieve a probe on a planet where things at the poles weigh 600 times the amount at the equator.
Starlight - 1971 printing - Alien explorers on a heavy-gravity planet
The Nitrogen Fix - 1981 printing - Earth's atmosphere changes from oxygen to carbon dioxide. Humans survive wearing masks.
Cycle of Fire - 1975 printing - a human and an alien, both castaways on a desert planet, work together to survive
Space Lash - 1969 printing - short stories
Still River - 1987 printing - Students research a cave-riddled planet that has less mass than it should.
Close to Critical - 1964 printing - aliens on a planet with a condensing atmosphere
Close to Critical - 1981 printing - aliens on a planet with a condensing atmosphere
Needle - 1967 edition - an alien takes over the mind of a human
Through the Eye of a Needle - 1978 edition - an alien forms a symbiosis with a human
Iceworld - 1977 printing - (two copies) - high-temperature aliens trade with humans for tobacco and other narcotics
Ocean On Top - 1973 printing (two copies) - undersea cities on Earth
If you'd like these, please comment. If multiple people want them, I'll give preference to the ones with lowest postage costs, and then flip a coin. If postage costs would exceed the cost of picking them up at your local used bookstore, I'll find a home for them locally. NOTE: US Customs requires me to get the full legal name, home address, and telephone number of recipients in the US.