This is the thirteenth Discworld book, a stand-alone story set in Omnia. It tells the story of Brutha, a trainee monk who has a photographic memory and the Great God Om, who is currently in the form of a tortoise.
The Omnian religion is one of fire and brimstone. It has a Quisition that kidnap people and torture them for believing in gods other than Om, or for believing that the world is flat. Both of these things are true in the Discworld.
The Quisition is run by a man named Vorbis, who is utterly ruthless. He is planning a diplomatic journey to the country of Ephebe, and decides to bring Brutha along as he thinks his memory will be useful. Of course, Vorbis is scheming and lying, and he plans to overthrow Ephebe.
During this time Brutha is communicating with the God Om in his tortoise form, he is the only one who can hear him. Om has lost his power and is determined to get it back. It becomes clear that the reason he is a tortoise, and the reason that only Brutha can hear him is because he is the only person left who truly believes. Everyone else believes in the church, or the rituals, or simply fears the Quisition.
Clearly Pratchett is using a metaphor hear for real world religions and faith, and over the course of the book says quite a lot about humans and how they work. I thought this was a pretty great book.