I found this book after seeing it recommended by Lin Carter as being a good post-Tolkien, Hobbit-like children's' story. In fact; I found its (inferior) sequel first, in the Bristol City Library - and then afterwards bought a paperback copy of The Minnipins (also called The Gammage Cup in the US edition).
It has proved to be an enduring favourite of mine, which I have re-read multiple times over the decades - mostly recently last week.
The Minnipins is an nigh-perfect children's fantasy; and deserves to be considered one of the classics of the genre.
It ticks all of the boxes: assured and convincing world-building of a charming and believable society; likeable and distinctive characters; original and sinister baddies; good writing - prose and folk-rhyming - spanning from excitement through humour, lyrical beauty, with touching emotional scenes; and a very well-structured plot that moves through several interesting phases to a satisfying conclusion.
Perhaps what raises The Minnipins is its memorability. Some of the rhymes and 'maxims' (used for chapter headings) really stuck; and some of the scenes became vividly established in my imagination on an apparently permanent basis.