Small Gods
The great god Om returns to his mighty church and finds himself trapped in the body of a one-eyed tortoise, with exactly one believer left: Brutha, a novice who cannot read but never forgets anything. A standalone Discworld novel about belief, institutions, and what happens to a religion when the god goes missing. Widely held up as Pratchett's best single book.
It is still a Discworld book: the footnotes wink, the names are jokes, and the comic engine sometimes idles when the argument wants to run. Readers allergic to whimsy will bounce off the talking tortoise long before the theology lands. The villain is efficient rather than deep, the ending is kinder than the middle has earned, and satire of organized religion is a large barn to hit with an entire fantasy novel.
The case for it and the rest of the canon open with Pro.
