From the Wikipedia list of libraries damaged during World War II, I learnt that millions of books stored in libraries were lost from bombings and intentional destruction by the Nazis, especially in Poland. However, in the list of literary works during the 20th century, we only see a few listings related to the war. So I wonder which one is more true: that we don't know what was lost among the millions of works which perished during the war, or that most of the books destroyed existed in many copies and are still extant?
Edit: This question has been closed and I am asked to clarify. So let's focus on the destruction of Warsaw. The argument makes sense that the Nazi book burnings in Germany would largely be inconsequential in terms of literary loss because the books targeted had been sufficiently widely published to avoid being completely wiped out. But this is what I've read about books during the destruction of Warsaw:
Unlike earlier Nazi book burnings where specific books were deliberately targeted, the burning of those libraries was part of the general burning of a large part of the city of Warsaw. This resulted in the disappearance of many valuable old books and scrolls among about sixteen million volumes from National Library, museums and palaces burnt indiscriminately by Germans in Poland during World War II.
The National Library lost at least 39,000 manuscripts – and most likely more, perhaps as many as 50,000 – along with some 80,000 books from the fifteenth to eighteenth centuries, 100,000 books from the nineteenth and twentieth centuries, 60,000 drawings and engravings, 25,000 musical scores and 10,000 maps. The great family libraries were almost wiped out, such as the famous collection of manuscripts of the Krasiński Library, of which just 78 volumes survived out of more than 7,000. Few of the most valuable Warsaw collections could be saved. The nineteenth and twentieth-century collections of the National Library housed in the SGH building on Rakowiecka Street were somewhat more fortunate.
Obviously the loss was beyond devastating, no matter what. But my question is this: how much information loss resulted from the destruction? Sure, millions of books and tens of thousands of manuscripts were annihilated, but could we estimate how many among these were unique copies, the annihilation of which has resulted in irrecoverable information loss to the humanity?
Related: This document prepared by UNESCO in 1996 tries to list all the libraries and archives around the world destroyed in the 20th century.
This project by Dr. Krista A. Murchison at Leiden University attempts to document and recover the medieval manuscripts destroyed in the Netherlands, Belgium, France, and the UK during WWII.
Two journal articles on the destruction of Jewish books in Poland: "Embers of the Soul: The Destruction of Jewish Books and Libraries in Poland during World War II", "The Destruction of Jewish Libraries and Archives in Cracow during World War II".