I have not read any Terry Pratchett so this review would probably focus more on Gaiman. However, let me be clear that this books is real collaborative writing. The edition that I have begins, quite amusingly, with an interview. One that is answered jointly by a Gaiman-Pratchett entity. It claims that the way the book got written, no one really knows who wrote what part. Sometime Gaiman would commend Pratchett on how wonderful a line he had written and as it would turn out, it was Gaiman who had actually come up with it over a phone conversation! So much for the saying that writing is a solitary art.
In a hilarious turn of events Crowley and Aziraphale seek out to prevent the anti-christ from meeting the four horsemen from hell and avert the apocalypse. In a beautiful satire on contemporary religion, politics and human society, Gaiman and Pratchett drive home the message – that humans are ultimately responsible for their own sins – clearly and effectively.
The book is a laughter riot. There is witticism in every sentence and quite often it actually makes you laugh out loud. A must read!