John Kessel
 
 

Wells, H.G.
The Island of Dr. Moreau

I know it's famous, yet it is nowhere near as well known as The Time Machine or The War of the Worlds, though it's probably Wells's best novel.  Book four of Gulliver's Travels transformed into a blasphemous evolutionary parable.

1930s vision of a tribalistic future in post-apocalypse England, starkly and humanely written.
 

Wylie, Philip
Finnley Wren (not really sf, but so what?)

Another lost masterpiece of the 1930s, a novel in the manner of Tristram Shandy, full of misogyny, wit, a terrifying description of a forest fire, and one-and-a-half sf stories.
 

Tucker, Wilson
The Long Loud Silence

As much as he is known as a fan, Tucker is vastly under-appreciated as a serious writer. In the 1950s he wrote a series of calm, unsentimental books about ordinary men facing extraordinary circumstances. Try this one, and then find The Lincoln Hunters and Wild Talent.
 

Golding, William
The Inheritors

The fate of the Neanderthal at the hands of Cro-Magnon. The anthropology may be dated, but the story devastates.
 

Knight, Damon
A for Anything

Knight at his most incisive punctures half a dozen sf cliches, but the power of the book is its bleak picture of the hero's submission to whatever rules.
 

Simak, Clifford
They Walked Like Men

Though I might have chosen Simak's Hugo Winning but still forgotten Way Station as an alternative, I enjoyed this book more than almost any I read in the early sixties Uncharacteristically close to Phil Dick territory, about aliens (they look like bowling balls!) who can assume any human shape and are buying the world. Funny, horrifying, with hints of creepy sexuality.
 

Disch 334

Overshadowed by Camp Concentration, this is Disch's best book, poised on the edge of satire, preceding his descent into epicurean cynicism.
 

Robinson, K.S. Pacific Edge

The least noted of Robinson's California trilogy contains his most gripping human story, of frustrated love. And the guts to base a plot on a zoning battle.