First – I finished the book at a reasonable time last night and then stayed awake until 3 am even with a couple of Tylenol PM at 11 pm – whatever –  I watched the interview (below) this morning.  

First –  I LOVED THIS BOOK!!!!!  

“An extraordinary ‘non-fiction novel’ weaves a web of associations between the founders of quantum mechanics and the evils of two world wars.”
(From the publishers, but “non-fiction novel” was originally from John Banville in the Guardian  – the whole quote is found in several “reviews. And at: https://en.wikipedia.org/wiki/Benjam%C3%ADn_Labatut

And Labatut said “…it is a book made up by an essay (which is not chemically pure), two stories that try not to be stories, a short novel, and a semi-biographical prose piece.” 
(Ibid)

 I’ve never quite known what to think about “nonfiction novels” since I first heard that in connection with Truman Capote’s “In Cold Blood” (1966) which I read when it came to my library’s Recently Released shelves. There are a quite a number of mixed genre books like  this though and Capote’s was by no means the first.   

Labatut addresses this seeming contradiction in a very educational and entertaining interview with Lawrence Weschler at: 
https://tinyurl.com/3atvpdat
( https://www.nyrb.com/products/when-we-cease-to-understand-the-world?variant=37890166784168 ) 
I think they used Zoom and the backgrounds for the two speakers are outstanding – each man is in his nest surrounded by books  

And it is a superb interview – one of the best I’ve ever seen.  Labatut is a hunk and they’re both pretty smart and funny. I was as glued as I get these days. 

Imo, about fiction/nonfiction, we have the former which is pretty much all imagined.  Then we have the latter which is either verifiable info via reliable sources or it’s some good critical thinking stuff.  In between we have “based on the real story” and “creative non-fiction” and finally the “non-fiction novel” in the order of how much imagination is involved.  I’ve read all those kinds plus memoirs which are often a blend and there are probably more – . 

So imo, “When We Cease To Understand The World” is indeed a non-fiction novel.
It’s a “novel” in that the stories are pretty much in chronological order from 1868 through today (2020) with some overlap. The work has integrity. It has balance. It has a story arc with increasing tension as you get further along in it.  

Also because although it’s indirect and over space and time, the characters do affect each other. And finally there is a plot – The overarching plot is the development of scientific/mathematical concepts from almost purely material, observable and measurable to almost purely abstract.  I’d say the main theme is “what happens when “We Cease to Understand the World.”  

The basics in the story line (and that’s probably 90-95% of the book) are all verifiable, but there are important parts which were imagined by Labatut. 

Story 1 PRUSSIAN BLUE involves Fritz Haber (1868-1934) who was involved in the extraction of nitrogen from the air and the mass production of artificial nitrogen and Zyklon B a pesticide used in the death camps of Germany). Also mentioned here are Johann Conrad Dipple (1673-1734), Carl Wilhelm Scheele (1742-1786) who used Prussic acid to accidentally discover/create cyanide (1782),  “Adi” (Adolf Hitler- 1889-1945) of Landsberg prison and 

Story 2 – SCHWARZSCHILD’S SINGULARITY is primarily about Karl Schwarzschild (1873-1916)  who polished Einstein’s theory of relativity with an exact solution to some questions. 

Story 3 THE HEART OF THE HEART is about the early math behind a lot of this – Shinichi Mochizuki plays a part but it’s mostly about Alexander Grothendiek (1928-2014), one of the most important mathematicians of the 20th century. He wanted to find and understand the foundations of mathematics. There’s a nice but abbreviated biography of him here. He followed mathematic abstraction as far as he could (near insanity) and then, after witnessing US bombing in Vietnam, believed scientists would destroy the earth so he forcefully turned to the environmental issues of our day living out his life as a very eccentric recluse. 

And WHEN WE CEASE TO UNDERSTAND THE WORLD is the novella. The names here are Erwin Schrodinger (1887-1961) and Werner Karl Heisenberg (1901-1976). And now there was a battle in the quantum world. Were elementary particles  waves (Shrodinger) or  “something dark at the heart of things.” (Heisenberg).  Prince Louis-Victor Pierre Raymond and his elder brother, Maurice de Broglie, a physicist come into the picture. And this is where Einstein comes in for a minute and when he says his now famous words, “God does not play dice with the universe.”   He also brought along with 

And the last story, THE NIGHT GARDENER, skips up to today and the 1st person may be Labatut himself having his own experiences.  
https://en.wikipedia.org/wiki/Benjam%C3%ADn_Labatut