Encounter Books / Quadrangle Paperbacks

Quadrangle Books Begins Quality Paperback Line, PW, July 1, 1963, at 41 (announcing Encounter Books, the first 6 titles of which are to be published on Sept. 6, 1963).

PW, Nov. 11, 1963, at 79:

QUADRANGLE BOOKS of Chicago, which originally started publishing paperbacks under the series name Encounter Paperbacks, has changed the name to Quadrangle Paperbacks to establish a closer identity with the parent house. From now on all the firm’s paperbacks, including the first six, already published, will be referred to as Quadrangle Paperbacks and will carry a QP code number.

Note: QP1-QP98 was the initial series. It's unclear if QP94-QP98 were actually published as Quadrangle Books, as they came out in 1972, right around when the Quadrangle series was dropped by The New York Times Company. All these series overlapped in chronology, to an extent (some came later in the 1960s.)

Titles from QP101-QP116 dealt with philosophy, in particular phenomenology.

QP201-QP242 seem to be about American social issues.

QP301-QP313 appear to be about mid-century European history, in particular the Holocaust.

QP350 - QP351 were a small sub-series called Modern Scholarship on European History

QP401-QP413 is the A History of American Life series.