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.
A Short Stay In Hell stuck with me to some degree ever since I've finished it. It's somwhere around 100 pages long if I remember corecctly.
It's quite disturbing but it, somewhat ironically, helped me put things into perspective back when I've read it.