I'm not sure about "essential reading", but I like most anything besides Science Fiction or Fantasy (probably the opposite of most folks here)

Some of my top choices would be:

Anything by Bulgakov (Heart of a Dog and Master and Margerita are both pretty widely available in English)

Berlin Stories by Christopher Isherwood

Burmese Days by George Orwell

Cakes and Ale by Somerset Maugham

Color of Summer by Reinaldo Arenas

Williwaw by Gore Vidal

The Big Sleep by Raymond Chandler