Lord of Light, Roger Zelazny. Brilliant, prescient, and genuinely a great work of literature all at once. The story of Rild, the telling of the metaphor about fire, so much else, it's been all these years and I'm still quoting it.
Bridge of Birds, Barry Hughart. When my will to go on falters, this is one of the books I turn to for comfort. It's beautifully written, it's hilarious, and it just makes me feel better.
Callahan's Crosstime Saloon, Spider Robinson. I genuinely have handed this book to a troubled young person and had them find a better understanding of the human condition between its covers. I didn't expect that, I thought I was sharing a cool book with them that was something I'd found influenced how I am, but it happened. It's kind of a big deal. It's also actually a lot of fun to read, it's just a collection of short science fiction stories set in a bar, right? ...right?
Why I Left Harry's All-Night Hamburgers, Lawrence Watt-Evans; Watt-Evans is largely a moderately obscure (as far as I can tell) fantasy author. I love the rest of his work because it's much more human than a lot of fantasy, with people who are bumbling and desperately trying to handle bizarre problems they're ill-equipped for and sometimes making their problems worse than they dreamed and also there are wizards. (I also like some of his worldbuilding choices, but let's get on with this). This one short story (that won a Hugo and stuff), though, lives rent-free in my head forever; it's got a simple point, which is that the world we're actually in has a lot of cool stuff, go enjoy it, but it makes it in a very fun way and, well, okay, enough, I love it.
Calvin and Hobbes. All of it. Bill Watterson is a visionary genius.
I can go on, I haven't mentioned Douglas Adams or Sandman or Transmetropolitan or fnord or ten thousand other things, but I have other things to do and should content myself with finite length.