Good Omens by Neil Gaiman and Terry Pratchett
It's important to bear in mind that bringing the son of Satan to earth in human form in order to launch a war between heaven and hell and wipe out the human race is very much a team effort. It takes a convent of Satanic nuns, angels, demons, and the Four Bikers of the Apocalypse (War, Famine, Death, and Pollution... when penicillin was discovered, Pestilence threw up his hands and quit in a huff).
At the same time, trying to stop the Apocalypse are an angel and a demon stationed on earth who have, over the years, become rather good buddies, a band of small children, a witch-finder, a witch, and a handy little book of nice and accurate prophecies written by one Agnes Nutter, an exceptionally batty Dark Ages prophetess.
In addition to being a ripping good yarn, and gut-bustingly funny besides, this book makes me wish that Neil Gaiman had written about twenty more novels that I could run out and buy right now, and makes me glad that Terry Pratchett has.
If you liked...: Douglas Adams, the funny bits of Tom Robbins books, or Jonathan Strange and Mr. Norrell by Susanna Clarke, and other works of fiction that aren't afraid to take a detour by way of the footnote, this book is for you.