This is my favorite of all the Discworld novels, and one that stands alone. It involves the theocracy of Omnia, and touches on true belief, prophecy, the terrors of freedom, and yes, moralilty. True morality, as a matter of fact. Brutha is one of the most moral characters to ever come out of the Discworld universe, and in this book he journeys from being an unlettered novice to someone with a very good grasp of human nature - and its capabilities.
In a world where gods are created and sustained by belief, what happens when people begin to believe in the structure - with all its attendant nastiness - instead of the god? Well, in this case, you get a god who intended to incarnate as a bull and ends up trapped in a tortoise instead. And Brutha - the aforementioned novice - is the only one that can hear him, the only true believer left. Whereas Vorbis, the head of the Inquisition, is well on his way to being the next prophet of Om... and it seems that he truly believes he hears the voice of his god, when in fact it is only the echo of his own thoughts.
Vorbis is a fascinating character (he even has an audio format named after him!) His biggest triumph is that he gets other people to act like him - that is to say, he is exceedingly good at extending his murderous viewpoint. What's worse is that he truly believes this is something he needs to do to make the world holy. Such men are dangerous. Brutha - who has perfect memory - is extremely useful to him and his plans to dominate neighboring countries. He's also obedient, at least until he starts having thoughts of his own... thoughts which could lead to Brutha's ascendency to prophethood, or, if things go the way they're planned, to his death.
As far as Discworld novels go, this one stands alone. And the ending is the perfect capstone to a story of how even gods need to be moral.