The books Classical Banach Spaces III and Classical Banach Spaces IV by Joram Lindenstrauss and Lior Tzafriri never appeared (after having been promised in various places of volumes I and II).
As written by Albrecht Pietsch in his book History of Banach Spaces and Linear Operators, the reason the later volumes never appeared was that "the development was too vigorous. Thus, in order to finish this project, a complete rewriting would have been necessary". Even still, the influence of volumes I and II in Banach space theory has been exceedingly nontrivial; indeed, Pietsch also writes: "The two-volume treatise of Lindenstrauss/Tzafriri on Classical Banach Spaces has become the most important reference of the modern period".