It is hard to choose just one of Edward Gorey's works. Like many other fans, I stumbled on Edward Gorey though one of his collections (Amphigorey, etc), and was simply stunned at the macabre weirdness of it all. It looks like a children's book, it even sounds like a children's book, but the themes range from the surreal to the delightfully inappropriate.
Gorey, who was apparently a lovely, eccentric man who was invariably courteous to his younger fans, didn't just scar and titillate young fans through his own books. His art also appeared in dozens (hundreds?) of other classic titles, not least of which is John Bellairs' The House with a Clock in Its Walls. (If, like me, you grew up in the US watching Mystery! with Anglophile parents, you would've seen Gorey's work in the amazing opening sequence.)
Still my favourite, The Doubtful Guest (1957) is everything great about Gorey in 14 rhyming couplets. For children, there's not only the giggling wonder of the art, but it also encapsulates the lunatic world of grown-ups. This is a world where the intangible laws and by-laws of 'the way you're supposed to behave' prevent people from being brave, telling the truth and chucking out a (monstrous) houseguest. (We may even get a movie of it.)
Art by Edward Gorey