Caveats: it must be a single book, published on its own under a single title. Within, it can be a series of essays, or poems, or whatever.
I'm already anticipating many people adding the obvious ones: The Little Prince, Of Mice and Men, The Lorax, The Grinch, Uncanny X-Men Vol. 1 #141, etc.
Really, I'm curious about the obscure, rarely read, short books that caught you unaware — but LANDED.
e.g.
The first book of this kind, for me, was Indignez Vous! (Time for Outrage) by Stéphane Hessel. It is 32 pages. In its time, about 2011, it said a lot of things I needed to hear and hoped others wanted to say. If I remember it correctly at all, it's grown more relevant with time.
The latest book of this kind, for me, is the Burnout Society by Byung Chul-Han. Finishing it tonight, at 51 pages, I've re-read it twice already.
House of Stairs by William Sleator messed with my head for a while.
Among other aspects, I read it in - I think - the mid nineties. At one point one of the main characters describes a past activity - leaving her car to rescue another person, IIRC - and details how difficult was to do so because you have to put on a gas mask to be out in the atmosphere or something like that. (It's been most of thirty years since I read it, I apologize for being light on the details.)
At the time I was a kid and thought it ridiculous. After all, humanity had conquered acid rain and the hole in the ozone (though I don't recall if that had happened yet). Climate change would never get that bad.