I think it depends on the who I'm talking to, in general I'd recommend something like Hitchhiker's Guide to the Galaxy because it's really funny and easily enjoyable by a wide range of tastes. (I hate recommending media because my interests are usually bizarre/niche).
If I know that they have similar interests to me, I would recommend Philisophical Investigations by Ludwig Wittgenstein, it's an absolute classic that guides the reader through an incredibly wide range of thought-provoking ideas.
If I'm talking to someone with a strong background in single variable calculus and linear algebra, for sure I'd recommend Calculus on Manifolds by Michael Spivak. I don't like making sweeping statements all that much, but Calculus on Manifolds is probably the greatest multivariable calculus book ever written. Coming in at a total of only 146 pages, it is concise yet covers enough for a year long course. I've never seen such a great demonstration of pedagogy in any textbook, it's a staple, definitely something that will completely change how you view and think of mathematics if you go through it. Chapters 4 and 5 are especially exciting, the text builds up to the generalized Stokes' theorem which I think is just one of the most beautiful results in undergraduate math.