The Persian Boy -- Novel by Mary Renault

this is almost certainly what Wizard is referring to. It's an historical novel told from the perspective of a beautiful Persian boy whose father was on the wrong side of a conflict. The boy was taken into slavery but because of his beauty was castrated and groomed as a sex slave. He became a catamite first of Darius the Persian king and then of Alexander. He put up with Darius because he had no choice but he fell in love with Alexander as Alexander did with him. The conceit of the novel is that while the narrator was indeed the boy, he tells, as an older man, the story of his youth. The events recounted in the book (aside from the framing) seem to stretch from the boy's capture when the boy was about ten to his late teens. It was one of a handful of novels that nurtured BLs of my generation -- the others being Edmund White's A Boy's Own Story (again told from the perspective of an older man but he is recalling his affair in his mid teens with a 12 year old boy) and Thomas Mann's Death in Venice. There were other books too -- e.g., Somerset Maugham's "The Narrow Corner" -- I posted a review some years ago on Boywrite -- Christopher Isherwood's Good Bye to Berlin, but you had to "translate" as the authors were engaged in degrees of elf-censorship (as Isherwood later admitted in later recounting his affair with a 14 year old German boy, "Heinz.") SR