Fifteen Volumes

Before Poul Anderson's History of Technic Civilization was gathered into seven omnibus volumes, it had been spread across perhaps eighteen regular-sized volumes, whether novels or collections. Other authors, of course, have written linear series of comparable or even greater length. The central part of James Blish's more compact body of work comprises fifteen volumes including prequels, sequels and cross-references although without in this case combining into any single linear sequence:

The Star Dwellers and Mission To The Heart Stars are companion volumes;
The Quincunx Of Time and Midsummer Century are, very slim, companion volumes;
each of these diptychs is loosely linked to one other, shorter, work;
the two shorter works, "A Dusk Of Idols" and "A Style In Treason," were collected in Anywhen;
each of the diptychs also refers back to an earlier work featuring Adolph Haertel;
Mission To The Heart Stars summarizes the plot of "Common Time," collected in Galactic Cluster where it does form a linear sequence with two other stories;
The Quincunx Of Time refers, less directly, to Welcome To Mars;
one other novel, A Case Of Conscience, refers to Haertel and is Volume III of the After Such Knowledge Trilogy;
two parallel linear "tetralogies" are the pantropy series, collected in one volume, and the Okies series, collected in four regular volumes or one omnibus volume.

These fifteen regular-sized volumes contain six series with mutual cross-references and sometimes even overlapping contents. Since Volumes I and II of After Such Knowledge are, respectively, a historical novel and a contemporary fantasy, they seem to be far removed from the remaining thirteen volumes. However, the central character of Volume I is Roger Bacon, the founder of scientific method, thus an early predecessor of Blish's fictional scientists, including Haertel.