And Eternity - Piers Anthony

Jolie, the ghost wife of Parry, now Incarnation of Evil, lives in a drop of blood on the wrist of Gaea, the Incarnation of Nature. Jolie is tasked with observing mortals for their suitability as future Incarnations, on behalf of Gaea. Gaea sends Jolie to shepherd Orline, a mortal with close ties to several Incarnations, who has commited suicide after the death of her teminally ill baby, Gaw Two. Orline is determined to save the soul of her child who, through no fault of his own, will continue to suffer in the afterlife. Orline and Jolie embark on a quest to find Gaw's soul and ameliorate his suffering. They meet Nox, the ancient and powerful Incarnation of the Night - she has Gaw, and tells the travellers that they must obtain an item from each of the seven major Incarnations in order to restore Gaw to innocence (and therefore to Heaven).
Part way through their journey the women inhabit the body of Vita, a fifteen year old prostitute who’s starting to get hooked on Spelled H, a powerfully addictive and destructive drug that wreaks havoc on the soul. With the help and guidance of Jolie and Orline, Vita flees her pimp and winds up in the court of Roque, a truly good and principled judge. After hearing that the body of Vita is inhabited by two other women, one of whom has the ability to see by aura is a person is suitable for a particular post, or telling the truth, or being deceptive, Roque agrees to act as Vita’s guardian.
Sexually abused by her father, and having never met a decent man, Vita falls in love with Roque and, after a time and much protest from the judge (who secretly yearns for underage girls), they consummate the relationship.
But Orline’s quest is not yet over. She garners the sought objects, or promises of them, from each of the Incarnations, until she finally approaches the most powerful of them all, the Incarnation of Good. And it is then that she discovers what the secret plotting of Satan, the importance of Luna Kaftan, and the unprecedented interference by Nox in human affairs are all about. With humanity trembling on the verge of a precipice, the Incarnations are poised to take unique action in a rare moment of unanimity.
This is a truly satisfying ending to the series – the characters are coherent and well drawn, the ending is a welcome surprise that is clearly posted and obvious on retrospect, and the ends are convincingly tied up without stretching disbelief. – Alex