Much like the Harper Religious Department Books, this series is a little different from most others of the time, as they weren't necessarily unified by a common theme nor marketed in any particular way. The name itself is more or less a personal creation, as the only indicator that the titles belong together in a series is the use of a series number beginning with TD. That TD stands for Trade Department is simply an educated guess based on presumptions I made with the Religion Department Books and knowledge of Harper & Row's departmental organizations. It seems that titles were given a TD series number in paperback when they didn't fall into one of the more discrete series like Torchbooks, Colophon Books, or the Perennial Library.
The series has both a murky beginning and end, as the earliest series number I could find was TD 109 and the latest TD 305. The only source that really used the TD series numbers was Bowker's Publishers Trade List Annual, of which there are many volumes that are unavailable for browsing/searching online. Many of the unknown titles in the list could be found in the PTLA volumes, but the full list will probably never be known (if it even exists).