When Brother Brutha of the Omnian Church starts talking to a tortoise, he merely assumes that he has gone mad. However, when the tortoise turns out to be the great god Om who is having a Bad Day, Brutha finds that his faith is about to be put to the test...
Up to (and including) Witches Abroad, Terry Pratchett was an author who wrote books that broadly fell in two categories: books that spoofed or were a satire of modern society in some way, often through broad comedy, and other books that were a bit more serious and had a point to them, though still amusing. The two sides had come very close to coexisting in Pyramids, but arguably just managed to avoid fusing into one impressive whole.
Small Gods is where Pratchett got it right. The entire book, from its first page to its last, is a lengthy, sustained and inordinately clever examination of religion, fundamentalism and blind faith and their conflict with reason, argument and science. And you barely notice, because the story itself is extremely taut, well-told and brilliantly characterised with Pratchett's occasional bursts of silliness kept to a minimum in favour of flashes of wry and at times angry humour. Small Gods is a book that Richard Dawkins would kill to have written, and done so in such a manner that even the most God-bothering evangelical would have still been riveted to it.
Small Gods has the veneer of being just a traditional Pratchett book: there's some jokes about men in togas arguing pointlessly about philosophy (in a world where it is difficult to have a conversation about, "Are the gods real?" when a lightning bolt will come flying through the window five seconds later with a label attached saying, "YES,"), Death has a couple of cameo appearances and there is a running joke about tortoises being nice to eat. But you can tell the subject matter really got Pratchett riled up. His hatred of blind faith and the idea that killing people is okay because some book says so - and, let's face it, that book was written by a old guy who might have been bitten by a donkey that morning and was a really foul mood when he started on the bit about doing unto others with fire and brimstone and was probably not, when you get down to it, an actual deity - really comes through in this novel, but in measured tones.
Character-wise, Small Gods may be Pratchett's strongest book. Most of the cast does not reoccur in the series (Death and a very brief trans-temporal appearance by a certain simian book-collector aside), but Pratchett still has time to paint them in impressive detail. Vorbis may be one of the scariest 'villains' (if that's even a right description) in the whole series. Brutha is certainly one of its most interesting protagonists. Om's pragmatic, tortoise-meets-deity outlook on life is amusing. Even minor characters like Didactylos and would-be rebel leader Simony are well-rounded and given good rationales for what they do.
Almost as importantly, the ending does not suck. Pratchett has a patchy record with endings, with his books sometimes ending okay and others being a bit of a let-down after a strong start and middle section. Small Gods, however, has a fantastic ending, starting with possibly the biggest belly-laugh out of all thirty-odd books in the series (hint: it involves something being airborne which shouldn't be) and proceeding from there.
Intelligent but never preachy, philosophical but never boring, Small Gods (*****) is Terry Pratchett's masterpiece (okay, his strongest masterpiece). It is the strongest Discworld novel and almost certainly the best thing he has ever written.