Terry Pratchett has always pushed the idea that gods are only as strong as the number of their worshippers - it's an idea that's propagated through fiction, to be seen in not just Pratchett's work, but also Gods Behaving Badly by Marie Phillips (2007), K.V. Johansen's Blackdog (2011), and the entirety of Max Gladstone's Craft series (2012-2014). Although this raises an interesting thought for me: was this Pratchett's idea, or someone in the 1950s, or was it one of the Greeks in 500 BC?
In Witches Abroad, he extends the idea to stories: stories with a lot of people who are familiar with them are more likely to "be told" in the world ... and are also harder to derail. And so in this book there are fairy godmothers, glass slippers, girls in red capes, etc. as Granny Weatherwax, Nanny Ogg, and Magrat Garlick (unmarried, despite the antics in Wyrd Sisters). The three of them set off for foreign parts (a place that's one part Venice to three parts New Orleans) to see that a happy ending is achieved after Magrat inherits a wand that can't do anything except pumpkins.
Mildly amusing, but not up to the standards of my new favourite, Reaper Man. I admit this is in part because Granny Weatherwax - previously one of my favourite Pratchett characters - comes under close scrutiny and turns out to be less appealing than I found her in the previous book. Not a bad person (in fact, that point is closely examined), but neither as well written or as likable.