The Night Soldiers are spies of World War Two.
They are not a group of agents working together but an amalgram of the many men and women who fought, killed, sometimes died in the years leading up to and through the great war this world has ever seen.
Nationalities will vary greatly among the stars of these books. Soviet agents are plentiful and are often the good guys as they work to defeat the Nazis. British agents mingle with French resistance fighters often. Scandanavia and Baltic nations supply their own operatives as each country struggles for any branch they might grasp in the tidal wave of destruction that constantly grows throughout the entire continent.
This is not a series like all the others in this collection. It is instead a collection of its own, individual books dealing with various agents from various countries, stand-alone books that do not need any particular order to be read. There is no chronology to be followed for the books are in no particular order.
That is not to say that these books are totally unconnected to each other for there are many threads which loosely tie them together. First is the conflict they depict, WWII. Second is the region in which they mainly occur, Europe. Third is the fact that many, many characters are found in different books, usually in varying degrees of importance. Fourth are the locations that many of the characters frequent, not just in country or city but actual places like the Brasserie Heiniger, as an example, where people show up for many different reasons in different books.
A person might be the main protagonist in one book but a very minor character in another. Or he or she might be mentioned by someone in one book and by someone else in a different one.
Reading one character mention a place for a rendezvous can often make the reader nod and think of how someone else had a meeting there the year before or that is the same restaurant so-and-so is going to be shot near in a few months. These kinds of references help greatly to make the books, separate as they are, feel very much connected.
The one absolute connection they all have is that they are incredibly realistic, compelling thrillers.