This volume contains one Riverbank publication: Publication No. 22: The Index of Coincidence and Its Applications in Cryptography. The importance of the "index of coincidence" in modern cryptanalysis cannot be overemphasized.
Friedman broke new cryptanalytic ground with all of these systems, coming up with new attacks and utterly demolishing the Vigenere, which was at that time considered "unbreakable". He hadn't yet developed the science to the level he brought it to by World War II later in his career, but this is a valuable insight on the way his mind worked, and some of the earliest writing on the subject that he did.
In the first run, the works were printed in 8 small volumes -- but since the material was highly classified at that time, the printing run was only 400 issues.
After the army used them in their cryptanalysis course, they ascended into the heights of "rare-book heaven", and collectors and dealers trying to lay hands on one would move heaven and earth to get it.
I was very pleased to see that someone finally managed to get permission to reprint these volumes -- albeit in a different format and in only 3 volumes. Now at last I can own copies of this piece of cryptological history.