The Minnipins, by Carol Kendall (Called The Gammage Cup in the US apparently) and a sequel, The Whisper Of Glocken, by Carol Kendall.
The main theme in the book is conformity versus nonconformity. Kendall uses five characters who do not fit in with their society but are able show their worth in saving it. The New York Times Book Review called the book a "fable about conformists and non-conformists". A reviewer from the Black Gate said the book is "a warning against 1950's conformist tendencies".
Another theme used in the book is uniting against a common foe. Although the Periods and the outcasts are initially in opposition to one another, they are forced to reconcile their differences when they are all threatened by the Mushrooms. The theme of individualism are prevalent throughout the book. When Muggles is invited to return to the village by Ltd., she tells him she is no longer "poor simple Muggles" (184) and has gained individuality in her life at the knoll. Horn Book said that the book offers "insights into the value of individualism". Muggles' heroism makes the book appear feminist; Kendall neither confirmed or denied this, saying "I just write the way it is".
Another theme is a kind of military racism, or species-ism. The Mushrooms are clearly described as different, although they would seem to be human, to some extent. However, when it comes to battle, the Mushrooms are treated like non-humans or animals—alien. Killing them is necessary, impersonal, and raises no questions of conscience. The Minnipins feel no pity for their enemy, even as the story describes the shrieks from the Mushrooms as they are killed.