Harper

1962 (May 1) - Harper & Bros. merges with Row, Peterson & Co. to form Harper & Row
1969 (March) - Harper & Row buys Basic Books
1977 - Harper & Row opens a San Francisco office

Harper & Row ISBN Prefixes (from 1984 PTLA): 

• 0-06 - Harper & Row

• 0-100  - Abelard Schuman/Criterion

• 0-309 - Funk & Wagnalls

• 0-381 - John Day

• 0-397 - J.B. Lippincott Co.

• 0-465 - Basic Books, Inc.

• 0-690 - T.Y. Crowell and Lippincott & Crowell

• 0-87154 - Russell Sage Foundation

• 0-88410 - Ballinger Publishing Co.

Torchbooks / Colophon Books / Perennial Library

Harper's paperback lines had some level of overlap in titles and scope in their 1960s heyday. The Torchbooks line, while initially marketed to a religious audience, quickly expanded its gambit to include science, academic, fiction, and other areas. With the introduction of the University Library sub-series in 1962, into which nearly all Torchbooks titles were funneled by the late 1960s, its primary audience became the classroom, and titles in the series were offered at short discount. Books on religion that were not directed at the classroom market were published in the generic, loosely-defined Harper Religion Department Series, which began in 1968, the year after Harper ended most of its Torchbook sub-series. 

Colophon Books had begun in 1962 with a general readership as its intended audience, meaning most purchases were through trade retail. Titles from the Torchbooks line that were considered to be of general interest were later issued as Colophon Books to capture that market. 

The Perennial Library, smaller and less expensive than the other lines, was directed at the secondary and university market, at least initially on its debut in 1964, although it later incorporated mystery and more general fiction titles. Titles from the Torchbooks and Colophon lines that could be sold to the secondary classroom were filtered down to the Perennial Library line during the late 1960s, sold at lower cost in the smaller, mass-market format. In the mid-1980s the Perennial Library subsumed most of Harper's general trade paperback titles, which had previously during the 1970s been issued in a generic, loosely-defined Harper Trade Department series.  

It's possible the changes in Torchbooks sub-series, with most of them ending in late 1967, has to do with Cass Canfield, Jr.'s ascension to publisher of paperbacks at Harper. Need to verify this by looking at exactly when the lines were dropped. It's the following spring (1968)  that the RD books started, which I would guess is directly a result of Canfield's new position. The TD books seem to have TD numbers only starting in 1972, but the earliest title with a TD number is in the TD 100s, leading me to believe that that loose series actually began around 1968 when Canfield took over, regardless of whether trade paperbacks from 1968-1971 actually bore TD numbers.  

See also Eugene Exman, THE HOUSE OF HARPER: ONE HUNDRED AND FIFTY YEARS OF PUBLISHING 298 (Harper & Row 1967):

In the field of supplementary or assigned reading in the universities, paperbacks have made and continue to make an important contribution. Material of this sort is supplied by Torchbooks and Colophon Books, the two quality paperback lines, with presently almost 700 titles available, the bulk of which are reprints of established scholarly works in various fields. Perennial Library, the latest paperback line, is lower-priced and aimed at school and college students with the emphasis on literature. Cass Canfield, Jr., is the publisher of these three paperback lines, which for the next fiscal year should produce net sales of nearly three million dollars.

See also Cass Canfield, UP AND DOWN AND AROUND: A PUBLISHER RECOLLECTS THE TIME OF HIS LIFE 231 (Harper's Magazine Press 1971). 

It was during these years that Harper's became preoccupied with starting its own line of paperbacks to supplement our indirect interest in paperback publishing through stock ownership in Bantam Books. Melvin Arnold, then a religious-book editor under Eugene Exman, undertook to develop Torchbooks. an attractive-looking and carefully chosen series designed to appeal largely to college and graduate students. Torchbooks achieved immediate recognition and success and two other paperback lines were soon added: Perennial and Colophon, both of them aimed at a less scholarly and younger audience than Torchbooks. Although Perennial is now a fast-selling line, it ran into serious difficulties after a year or two due to overproduction, and when Arnold assumed general management responsibilities for Harper's, Cass, Jr. was chosen for the difficult task of getting our paperbooks back on the rails. He achieved this with firmness and skill so that Torchbooks, Perennial, and Colophon books, the three Harper paperback lines, are now very profitable.