I nominate The Tale of Genji
by Murasaki Shikibu.
Genji, the hero of the Tale, is the son of the emperor and his favourite concubine, Kiritsubo. A Korean sage predicts a brilliant future for Genji but his mother suffers the jealousy of rivals at court, becomes ill and dies. The distraught emperor becomes obsessed with the tragic story of Yang Kwei-fei, but eventually finds another concubine, Fujitsubo, who reminds him of his former love.
Since Genji lacks backing at court, the emperor makes him a commoner, assigning him membership of the non-royal Genji clan. The eldest son of the emperor and Lady Kokiden is made crown prince.
Genji becomes an uncommonly handsome and gifted young man, admired by all but feared by Lady Kokiden and her family. The first part of the Tale follows his amorous exploits with a variety of ladies in and around Heian-kyo, his friendship with To no Chujo and arranged marriage to To no Chujo's sister Aoi, the birth of his son and his budding relationship with the young Murasaki.
I will second Paradise Lost
. I've always meant to read that, but the in poetry part put me off.