[knivesout]

I Am Legend by Richard Matheson certainly qualifies, although it is as much horror as sf. If you like Wyndham's books, it seems you'd like the Matheson one too. Through Darkest America by Neal Barrett, Jr is another good post-apocalyptic novel. Walter M Miller's A Canticle for Leibowitz is a true gem of its kind and essential sf reading in general, too.