The Charioteer by Mary Renault (published in the 1950's).
A memorable book, one of the handful about gay men from the first half of the twentieth century, and one of the ones most worth reading.
Many people who have read her later historical novels of Greece must pick this up and assume from the title that it's going to be another in the series. In fact, it's her last novel of contemporary British life before she got impatient with having to write about people living under such heavy social restraints, and switched to a different style and a different period, when she believed gay men at least were more accepted.
Besides being extraordinarily convincing as a story and a presentation of the characters, it's unusual among earlier gay novels in not being primarily about homophobia. The main characters do have to hide parts of their lives, but they don't feel they have to marry and they are not the victims of violence, nor do any come to a bad end.
While much in the historical novels comes from her scholarship, this novel comes from her direct knowledge of the society she lived in, with her experiences as an army nurse during the war giving verisimilitude to all the parts in the hospital. It's as much a period piece (giving an intense sense of wartime life) as her later novels.
It's also unusual in involving a male triangle of sorts and a choice, but without a Mr Wrong and a Mr Right.
Some American readers will find parts of certain chapters difficult, like the one telling of Laurie's time at a private school, simply from our not being totally familiar with the cultural expectations which are so obliquely alluded to. Rereading some chapters afterwards gave me more confidence that I was picking up the meaning she intended, here and there. But you can have some doubts and questions and still be swept along. Eventually context explained a lot of things. And a lot of people with quick intuitions will have no trouble at all.