A series of seven books by Piers Anthony. I haven't read anything by him besides these, but apparently they're among his better works. Each volume deals with a particular supernatural office (or incarnation) in a well-crafted sci-fi/fantasy world. They are:

• On A Pale Horse (Thanatos, The incarnation of Death)
• Bearing an Hourglass (Chronos, The incarnation of Time)
• With a Tangled Skein (Clotho, Lachesis, and Atropos ]The incarnations of Fate)
• Wielding a Red Sword (Mars, The incarnation of War)
• Being a Green Mother (Gaea,The incarnation of Nature)
• For Love of Evil (Satan, The incarnation of Evil)
• And Eternity (God, The incarnation of Good)

All are good, and worth reading. I've heard that even people who, as a rule, dislike Piers Anthony, tend to like these books. They are, in as much objectivity as I can muster, well-crafted, thought-provoking fantasy. And a pretty quick read, to boot.