Good Omens by Terry Pratchett and Neil Gaiman

There is a hint of Armageddon in the air. According to the Nice and Accurate Prophecies of Agnes Nutter, Witch, the world will end on a Saturday. So the Armies of Good and Evil are massing, and the world’s last two remaining witchfinders are getting ready to Fight the Good Fight. Atlantis is rising. Frogs are falling. Tempers are flaring, and everything appears to be going to Divine Plan.

Except that a somewhat fussy angel and a fast-living demon are not particularly looking forward to the coming Rapture. They’ve lived amongst Humanity for millennia, and have grown rather fond of the lifestyle. So if Crowley and Aziraphale are going to stop it from happening, they’ve got to find and kill the AntiChrist (which is a shame, really, as he’s a nice kid). There’s just one glitch: someone seems to have misplaced him.