The Married Man by Edmund White
When the self-obsessed, Austin, an HIV positive American, living in Paris but on the verge of being an “old man”, falls for the younger Julien, he is stunned when his feelings are returned and they end up living together. But soon it is discovered that Julien is dying from AIDS and as his condition worsens (while Austin remains healthy) their relationship is pushed to its limits. This is a haunting tale of commitment and caring under the worst possible circumstances and how it sometimes takes a person a whole lifetime to grow up enough to really love someone.