Jeffrey St. Clair, the editor at http://www.counterpunch.org/, has published a list of his 100 favorite non-fiction books (originally in English) from the twentieth century:

100 Best Non-Fiction Books of the 20th Century (and Beyond) in English
(chosen by Alexander Cockburn and Jeffrey St. Clair)
5 April 2014
http://www.counterpunch.org/2014/04/04/100-best-non-fiction-books-of-the-20th-century-and-beyond-in-english/

I have read several of these books as well as a few others, not listed, by some of the authors cited.

Given Cockburn’s and St. Clair’s literary and political interests, they did not consider books on science like the physics book

Taking The Quantum Leap, by Fred Alan Wolf (1981, revised 1989, a National Book Award Winner),

nor scholarly works on folklore and Eastern thought like:

The Hero With A Thousand Faces, by Joseph Campbell (1949), and

The Philosophies Of India, by Heinrich Zimmer (1951).

Let me suggest a few more titles for your consideration, along this theme of “20th century books in English”:

Cadillac Desert, by Marc Reisner (1986, revised 1992).
For me, this book ranks with Thucydides.
I described Cadillac Desert, along with some other works, at
https://manuelgarciajr.com/2014/03/07/living-in-cadillac-desert/

At War With Asia, by Noam Chomsky (1970).
I described my reaction to reading this book, here:
https://manuelgarciajr.com/2012/06/20/on-reading-at-war-with-asia-by-noam-chomsky/

The Making Of A Continent, by Ron Redfern (1983).
Redfern tells, in words and with many photographs, how the Cadillac Desert that captivated Edward Abbey and Marc Reisner came to be (along with the rest of North America).

Vietnam, Inc., by Philip Jones Griffiths (1971).
A book of searing photojournalism that undoubtedly helped to end that war, by working on the public mind.

The Shock Of The New, by Robert Hughes (1981).
A fascinating presentation and explanation of modern art.

The Selfish Gene, by Richard Dawkins (1976).
A 30th anniversary edition of this seminal work on modern (DNA-based) Darwinism is in print.

A book written in the 19th century that was only published in its unexpurgated form during the 20th century is:

The Autobiography of Charles Darwin, edited by Nora Barlow (1958).
“The autobiography of Charles Darwin, 1809-1882, with original omissions [usually about religion] restored, edited and with appendices and notes by his granddaughter Nora Barlow.”
See the following blog entry for details about print and free online editions of this book
https://manuelgarciajr.com/2014/03/07/the-life-and-ideas-of-charles-darwin/

Three books (in English) that are 21st century publications (technically outside our 20th century time frame) but are entirely focused on the 20th century (and largely written during it) are the following.

Written by Tony Judt (1948-2010):

Postwar: A History Of Europe Since 1945,
(2005; writing began in Eastern Europe in 1989, during the year of revolutions).

Reappraisals: Reflections On The Forgotten Twentieth Century,
(2008, republication of essays that first appeared in journals between 1994 and 2006).

Thinking The Twentieth Century,
(with Timothy Snyder, completed 2010, published 2012).

Other 20th century books that were originally non-English, but are essential cultural artifacts are:

Relativity, by Albert Einstein (1879-1955)
(1916, expanded and revised up to 1952); written for the general public.

Most of the works by Primo Levi, like:

Survival In Auschwitz,

The Reawakening,

The Periodic Table
(a memoir with chemistry),

The Drowned And The Saved,

all written and published between 1945 and 1987.

These are the books that first came to my mind after reading Jeffrey St. Clair’s “top 100 books in English” article.

Some of these books may be more political while others are more scientific or ecological or artistic or philosophical or psychological, yet I think they all help illuminate facets of the collective consciousness of alert and concerned late 20th and early 21st century minds.

Enjoy!