I nominate "Flatland," by Edward Abbott.
A fascinating science-fiction story with some biting, funny satire of Victorian society, Edwin Abbott's Flatland still has a lot to say about modern life, mathematics, people, philosophy and our perceptions of reality. The story takes us to a two-dimensional world where all the inhabitants are flat geometric shapes, and who are all firmly convinced that "length and width" is all there is. But one enterprising shape discovers the existence of a third physical dimension, which leads to speculation about a fourth dimension - and that changes everything.
Abbott used the fictional two-dimensional world of Flatland to offer pointed observations on the social hierarchy of Victorian culture. However, the novella's more enduring contribution is its examination of dimensions; in a foreword to one of the many publications of the novella, noted science writer Isaac Asimov described Flatland as "The best introduction one can find into the manner of perceiving dimensions."
When I'm not here, I'm somewhere else. Mind the rainbows.