Perennial Library 4000 Series

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).