As an audiobook, this was extremely well read by Stephen Briggs. Kept me entertained while knitting. Good Omens is about the Anti-Christ, good vs evil and prophet witches.
Aziraphale and Crowley are 2 angels on earth who quite like humanity and all our screwed-upness. Aziraphale is the angel on God's side, while Crowley is the angel on Satan's side. In the thousands of years of working side by side, they've grown fond of each other. So when it is time for the end of the world, neither of them are happy and make a plan to help avert it.
The child who is thought to be the Anti-Christ isn't, thanks to a mix up at the hospital. The real AC is having a grand time growing up with a normal English family and normal friends.
Things kick into gear when the Four Horsemen (War, Famine, Pollution - Pestilence retired due to the invention of penicillin - and Death) start the end of times. Everyone is on the hunt for the AC, to either stop the Apocalypse or to help it along.
Entertaining fantasy by Gaiman and Pratchett.