The Charioteer by Mary Renault.

For those who haven't read The Charioteer:

Originally published in 1953, the novel is set during World War II in southern England in September/October 1940. Laurie is 23 and, although inexperienced, knows he is homosexual. His leg has been badly wounded in the evacuation of the British Army from Dunkirk, and he is still in hospital. At school he idolized Ralph Lanyon, three years older than him and the school hero. Ralph is expelled from school for a sexual encounter with another boy – of which he is actually innocent, but the intent was there.

In the hospital, Laurie falls in love with Andrew, a young conscientious objector who is working as an orderly at the hospital. Andrew, innocent and religious (he is a Quaker), does not realise the depth of Laurie's feelings for him, nor that he is reciprocating them, and Laurie does not wish to enlighten him for fear of losing him.

Laurie, going for physiotherapy in the nearest big hospital, meets up again with Ralph, who was also injured at Dunkirk. He discovers that Ralph commanded the ship which brought him back from Dunkirk but having lost half his hand when his ship was blown up, is no longer on active service but is still in the Merchant Navy. Ralph, rather to Laurie's horror, is part of the local community of 'confirmed' queers.

Ralph and Laurie are instantly attracted, memories igniting, and, when Laurie's widowed mother re-marries, become lovers after the wedding. Laurie is torn between Andrew, whom he can never have, and Ralph, whom he can have too easily. He is idealistic enough to believe that his love for Andrew can survive on platonic terms. In any case though, he will be leaving both of them behind soon when he is discharged from hospital and returns to Oxford for his final year at the university.

Laurie has made up his mind to chose Andrew over Ralph when Ralph's ex-lover, Bunny, who Ralph finished with upon Laurie's arrival, goes to see Andrew and tells him the truth about Ralph and Laurie's relationship. Andrew realises that he loves Laurie in the same way and, in expiation, goes to work in an area of London that is being badly bombed in the Blitz. He writes to Laurie and tells him that he cannot see him again. He believes that Bunny was Ralph.

Laurie, on receiving Andrew's letter, rushes up to London to see him, but in the end decides that it is better to let him go. He confronts Ralph with having gone to see Andrew. Ralph, knowing nothing about it, denies it, but Laurie does not believe him. Ralph realises that he is ultimately responsible and leaves.

Alec, a doctor in the hospital and also one of Ralph's ex-lovers, tells Laurie the truth and tells him to go round to Ralph's room before he does something drastic. Laurie arrives when Ralph is downstairs getting some postage stamps from his landlady. He reads the letter Ralph has written to Laurie before he commits suicide.

One of the themes of the novel is The Phaedrus of Plato. Ralph gave the book to Laurie before he left the school, and Laurie has carried it with him as his bible ever since. It contains the myth of the charioteer, in which Socrates likens the soul to a charioteer, driving two mis-matched horses, the white and the black, which pull the soul in different directions. Laurie leaves the book for Andrew when he visits London.

This is the ending of The Charioteer (chapter 16):

Ralph didn't move forward. His eyes were dragged down at the corners, as if with lack of sleep; he contracted them strainingly. Behind them, like an almost exhausted runner, his pride seemed to pause, to sway and balance. "What did you ring for?" he said. "I suppose you found out?"

Remorse, even the greatest, has the nature of a debt; if we could only clear the books, we feel that we should be free. But a deep compassion has the nature of love, which keeps no balance sheet; we are no longer our own. So in the presence of this helpless forgiveness, Laurie seemed to himself to be doing only what was nearest in the absence of time to think. There was something here to be done which no one else could do. All the rest would have to be thought about later. He looked Ralph straight in the eyes, believing what he said.

"Afterwards. Alec told me. But I should have come, anyway. I should have had to come back."

Quietly, as night shuts down the uncertain prospect of the road ahead, the wheels sink to stillness in the dust of the halting-place, and the reins drop from the driver's loosened hands. Staying each his hunger on what pasture the place affords them, neither the white horse nor the black reproaches his fellow for drawing their master out of the way. They are far, both of them, from home, and lonely, and lengthened by their strife the way has been hard. Now their heads droop side by side till their long manes mingle, and when the voice of the charioteer falls silent they are reconciled for a night in sleep.