Perennial Library 4000 Series
Publisher: Harper & Row / William Morrow
Related Series: American Heritage Paperbacks, ChapelBooks, Colophon Books, Harrow Books, Harper Perennial Classics, Harper Religion Department Books, Jewish Publication Society Series, Jubilee Books, Perennial Library, Torchbooks, Torchbook Library Editions
This sub-series consists of genre books, mostly mystery, and some appear to have been co-published by William Morrow in association with Harper & Row. There's not much information about the titles or why there's such a gap toward the end of the run.
News of Paperback Books--Summer--Fall: Harper & Row, PW, June 6, 1966, at 172-173 (announcing the publication of three Perennial Library titles being published by William Morrow in association with Harper & Row, with more to come).