The Persian Boy is a work of historical fiction by Mary Renault, published in 1972. It's the second book in her trilogy about Alexander the Great, following Fire from Heaven and preceding Funeral Games. However, it works perfectly well as a standalone novel.
The narrator is Bagoas, a young Persian slave boy (who was historically a real figure) in the harem of Persian king Darius III. After Darius’s death, Bagoas finds himself in Alexander's service. Through Bagoas's eyes, Renault tells the story of the latter part of Alexander's life, his relentless ambition and conquests, from roughly 330 BC until his death in 323 BC.
You feel for Bagoas, wrenched from his home at an early age, castrated, and forced into slavery. His initial years are traumatic and kindness is scarce. Nonetheless he resolves to survive, and eventually finds a compassionate and generous master in Alexander. Bagoas becomes Alexander’s devoted companion and lover, and his status as an outsider - as a Persian of relatively low status who gains intimate access to the king - gives the book a unique perspective.
Given how far and wide Alexander’s travels and conquests ranged, culture is a big part of the story as well. Having conquered the Persian world, Alexander then tries to integrate it with the vastly different Greek and Macedonian world, often to the dismay of his own men. Bagoas, who represents the more formal and hierarchical Persian world, is able to gently guide Alexander on Persian customs, and what might be expected of Alexander as their new ruler.
Renault portrays the relationships in Alexander's life with sensitivity, including his lifelong bond with Hephaistion. Bagoas is keenly aware of Hephaistion’s primacy in Alexander’s life, but despite his jealousy (which he never expresses except to himself), the two arrive at an unspoken understanding, united as they are by their love for Alexander.
Renault’s notes at the end of the novel indicate how meticulous her historical research was. Known for her sympathetic and nuanced treatment of same-sex love in antiquity, The Persian Boy is considered one of her finest works, a landmark of gay literature.