P 601-625 were evidently earlier reserved for the Harper Classics series, which ran from HC 601-621 (1965-1966).
A hardcover series called Harper Perennial Classics used the prefix HP- (see https://seriesofseries.owu.edu/harper-perennial-classic/).
A = $.50
B = $.60
D = $.75
F = $.95
Perennial Library: New Harper Paperback Line, PW, June 22, 1964, at 70:
Harper & Row, which already publishes a number of scholarly and specialized paperback series, will introduce one on July 29 which differs considerably from the others in being much more popular in appeal. The new Perennial Library titles will be major works of recent fiction and nonfiction, titles of general interest and of particular appeal to high school students. All books will be 4 3/16 x 7 1/8 inches, reset and redesigned for this edition, and will use a quality of paper superior to ground wood which is more frequently used in paperbacks. Covers will use a laminated four-color process, and the retail price will be from 50 cents to 95 cents, with a regular trade discount. Present plans call for the publication of eight titles to launch the series and four every two months thereafter.
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.
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.
Carolyn T. Anthony, The Great Trade Paperback Mystery, PW, May 21, 1982, at 36-40, 37 (in section titled "Perennial's Progress," reviewing Perennial line and introduction of mysteries to it, particularly four by Nicholas Blake published in 1977).
Harper & Row to Consolidate Paperbacks, PW, June 14, 1985, at 58 (noting that the Perennial Library "debuted as a low-priced, rack-sized paperback line emphasizing literature and academic titles aimed at high school and college students" but had recently undergone "considerable change, as rack-sized mysteries, as well as a wide range of literature . . . were added to the line.")
Harper Buys 'Billy Bathgate' for Reprint at $1 Million Floor, PW, Apr. 14, 1989, at 15 (mentioning, without remark, that Harper didn't plan to publish Billy Bathgate "as a Harper Perennial trade paperback").
'Simpsons' Creator Groening Moves from Pantheon to Harper, PW, May 4, 1990, at 33, 34 (mentioning in passing that the Perennial Library imprint had just been renamed Harper Perennial).