I doubt there is any single book we could suggest that would “sell the genre” to someone except by pure luck, just as there is no single book that would be certain to sell fine literature to someone. Fine literature and SF are both too vast in scope to be encapsulated in a single book. We don’t know what your story interests are (adventure, social drama, personal drama, comedy, etc.) nor what your narrative preferences are (encompassing plot, style, characterization, atmosphere, etc.).
If you want a near-future, hard science, survival story, you can’t do better than The Martian by Andy Weir (made into an excellent movie in 2015).
Next, I will mention my five favorite SF novels in no particular order after the first one. Earth Abides by George R. Stewart is the finest post-apocalypse story ever. But it is 317 pages, not short by my reckoning. A Canticle for Leibowitz by Walter M. Miller Jr., also a post-apocalypse story highly regarded by fans and academics alike, is considered the best SF novel dealing with religion (316 pp.). Much shorter are the next three.
Day of the Triffids by John Wyndham (219 pp, ) is a catastrophe story with great characterization (forget the idiotic 1963 movie; but the excellent 1981 BBC adaptation follows the book closely). Bring the Jubilee by Ward Moore (211 pp.) is a time travel story widely regarded as a classic. Its use of two levels of plot to illustrate the theme (that one can’t be merely an observer of life) is without peer. Roadside Picnic by Arkady and Boris Strugatsky (193 pp.), an awesome book in every way, is the oddest alien “invasion” story ever. Some unseen aliens briefly stopped on Earth and left behind a bunch of trash, which has hugely disrupted civilization. The only trouble with recommending RP is that the ending is extremely enigmatic; nevertheless, everyone in our book group (12-14 people) liked it.
I very much liked the Akutagawa Ryunosuke stories I’ve read, collected in Tales Grotesque and Curious (The Hokuseido Press, Tokyo, 1938) and Rashomon and Other Stories (Charles E. Tuttle Co., Tokyo, 1952/1983). I picked up those books because of my love of the Kurosawa film Rashomon. I’m also a fan of Hesse and found Demian to be a very interesting character study.