The books that I force upon people regularly are Richard Bach's Illusions
A lot of cool books have been mentioned on this thread but that's one I don't see recommended often. I read the much more famous
JLS first and it was neat but, though I don't have a copy at the moment, I read
Illusions a zillion times long ago. "You are quoting Snoopy the Dog, I believe?" - "I quote truth wherever I find it." The vampire parable illustrating the Platinum rule, so to speak. And da plane! Cool book. Very powerful towards the end, too. I read a later Bach or two and lost my fascination with Bach but I still think highly of that book.
As far as the topic and I go, I tend to try to be reader-specific and try to "hit the spot" though I sometimes will recommend books that might be on the edge and stretch boundaries, But I don't try to "convert" folks to things that I think they would be closed to.
So I'm not sure there are "go to" books that I recommend a lot. I do know that, if you can stand hard SF at all, I recommend Greg Egan's
Diaspora a lot because it's just really dense and vivid and imaginative but rigorous and just pushes all my hard SF buttons. Yet it's also fairly stylish - not in a way that would put off many hard SF fans, but might appeal to many who aren't. But if you're just purely allergic to hard SF then run screaming. He's very uncompromising. About the only Egan novel I know of that a non-hard-SF fan might enjoy is
Teranesia. That
is hard SF but done in a way that I think would be palatable to others - Egan doesn't generally skimp on characters or naturalist descriptions or the soft sciences even in the hardest stuff, but
Teranesia is much more of that than the other.
And, wherever I can, I recommend whatever Asimov is most applicable because I'm just a nut.
And the most applicable stuff from Henry Kuttner and C.L. Moore and others who are unjustly neglected or, say, Norman Spinrad who is underrated. And I always recommend short fiction at any opportunity because I love it and it's such a small component of things these days. Like Egan's
Axiomatic, and Asimov's
The Bicentennial Man and
The Best of's of Kuttner and Moore, and Spinrad's
The Last Hurrah of the Golden Horde.