As I mentioned in the review of "Witches Abroad," Pratchett was very fond of the idea that the power of a god is proportional to the number of its worshippers. This book draws the most heavily on that idea. Our main character is Brutha, a novice at the temple who incidentally is the only person left in the entire world who believes in the Great God Om. Om - because of his lack of worshippers - has found himself re-incarnated as a turtle, and unable to summon up a thunderbolt larger than a weak burst of static electricity. While Brutha is entirely unable to read or write, he does have an eidetic memory, and one of the most lethal high priests latches on to him to use this skill. So Brutha has to juggle the demands of his god and the demands of his priest. The "small gods" of the title are those who no longer have any worshippers, or who have never had any.
Pratchett is happily having a go at organized religion and its effects on politics. And once he warms to the subject, he takes a good swing at the Ancient Greeks and particularly their philosophers - although they don't take quite as big a hit.
Perhaps not Pratchett's best plot, it's nevertheless one of his funnier books: I enjoyed it considerably.