Terry Pratchett is a well-established English fantasy writer whose most famous work is the Discworld series.  Neil Gaiman is a bit of a literary Renaissance Man whose dabbles in a little bit of everything, from children's picture books to dark comics to fiction, all of it with a bit of a grim, Tim Burton-esque quality.  The two don't really strike me as particularly similar, aside from both being English and therefore having the British dry humour, but they blend seamlessly here.

Good Omens was published way back in 1990, when both writers were on the ascendant (have either come down yet?).   The story of the approaching End Times, we learn that the Antichrist everyone is expecting was switched at birth and is in fact a regular 11-year-old boy.  Also involved are various parties trying to hasten or prevent the end of the world are the remaining Four Horsemen (Pestilence retired at the advent of antibiotics), an angel and a demon (not your modern kind with the sullen expressions and pining for love, but the real classic versions.  Bad people.)  The plot is a bit tough to nail down, but if you've read either author's work, it a bit like that.

There are different versions, one American and one British, with mostly minor differences, and you should note that this cover I have posted is just one of many.  Good for boys comfortable with complicated plots, the story has plenty of jokes and Terry Pratchett's trademark footnotes to lighten the mood.