James Blish is one of the greats in science fiction. Cities in Flight is not even his best but still well worth reading.
Earthman Come Home was a series of stories fixed up into a novel. Triumph of Time was an original novel, a sequel to Earthman Come Home, albeit set centuries later. They Shall Have Stars is a prequel establishing the the origins of the tech in a near-future dystopia. And A Life for the Stars is a young adult novel set in the same universe. The hero of A Life for the Stars is mentioned in Earthman Comes Home. I recommend this order, or the chronological order, though, not Baxter's.
Some of Blish's best work was in short form, including the famous stories Common Time, Beep, A Style in Treason, Beanstalk, Sunken Univers, Surface Tension and How Beautiful with Banners.
Another series of novels was collectively titled After Such Knowledge, which is unique (I believe) in that each entry is in a different mode. Doctor Mirabilis is historical fiction about the life of Roger Bacon. It features one of the most stunning tour-de-force experiments in style I've ever seen, being written in a comprehensible pastiche of Middle English grammar. A Case of Conscience is Hugo-winning science fiction, about a Jesuit priest's encounter with an alien species, one member of which comes to Earth. Last, the doublet of short fantasy novels, Black Easter and The Day After Tomorrow, in which a magician releases the demons from hell.
Blish tackled ESP as a theme in the Fifties novel Jack of Eagles (also known as Esper, I think,) and the Seventies novel Midsummer Century. In both, Blish cleverly rationalized psychic powers, using entirely different rationalizations.
The Seedling Stars collects Sunken Universe and Surface Tension. These stories introduced the idea, so far as I know, of engineering humans for alien environments.
Blish also wrote science fiction criticism, second only to Damon Knight I would say, under the pen name William Atheling Jr.
The Star Trek adaptations are the least, most piddling of his work. His original Star Trek novel, Spock Must Die!, with all due apologies to the Trek writers who frequent this board, the only Trek novel I've ever seen that actually was intrinsically interesting as a novel.