The Persian boy / Mary Renault.

"Alexander the Great died at the age of thirty-three, leaving behind an empire that stretched from Greece and Egypt to India and a new cosmopolitan model for western civilization. In this stunning work of historical fiction, mary Renault vividly imagines the world of this charismatic leader whose drive and ambition created a legend.
The Persian Boy traces the last seven years of Alexander's life from the perspective of his devoted Persian lover, the eunuch Bagoas. Abducted and gelded as a boy, Bagoas was sold as a courtesan to King Darius of Persia, but eventually found his freedom with Alexander. Bagoas' extraordinary cultivation and loyalty mad him the perfect confidant for the great King. Their relationship strengthened Alexander as he weathered assassination plots, the demands of two foreign wives, a sometimes mutinous army brewing with factions, and his own ferocious temper. Bagoas was also witness to the depth of his King's friendships and hims commitment to his soldiers. Renault shows how this Persian boy (who is based on an actual historical figure) may have understood and supported Alexander's fervent designs for his empire better than any Macedonian general." -- Publisher's description.