The Third Love by Hiromi Kawakami

Having married her childhood sweetheart, Riko now finds herself trapped in a relationship that has been soured by infidelity. One day, by chance, she runs into her old friend Mr Takaoka, who offers friendship, love, and an unusual escape: he teaches her the trick of living inside her dreams. And so, each night, she sinks into another life: first as a high-ranking courtesan in the 17th century, and then as a serving lady to a princess in the late Middle Ages. As she experiences desire and heartbreak in the past, so Riko comes to reconsider her life as a 21st century woman, as a wife, as a mother, and as a lover, and to ask herself whether, after loving her husband and loving Mr Takaoka, she is now ready for her third great love.