It is probably a spoiler even to recommend it under these circumstances, but have you read Diana Wynne Jones' Archer's Goon (1984)?

I have, and I really liked it! Actually, now that I'm thinking about it, Jones really likes that trope. I can think of at least two other books of hers that have it (Hexwood and Time of the Ghost are the ones I'm thinking of, at least to an extent in the latter case).