Keystone Books, New Paperback Line, PW, June 2, 1958, at 119.
Ad, Publishers Weekly, June 23, 1958, at 14.
PW, Feb. 22, 1960, at 60:
KEYSTONE BOOKS, Lippincott's line of higher-priced paperbacks, is being expanded this spring to take in books outside the original categories of medicine and music assigned to the line. “New World Writing # 16" makes its first appearance in April under the Lippincott imprint as a Keystone Book. The original paperback anthology, formerly published by New American Library, is now produced by Lippincott editors Stewart Richardson and Corlies M. Smith and will be published semiannually (Forecast of Paperbacks, this issue). Other spring additions to the Keystone list are three popular backlist Lippincott titles. Marya Mannes' “More in Anger" (PW Jan. 18), Lionel Ruby's “The Art of Making Sense: A Guide to Logical Thinking" and “The Art Spirit” by Robert Henri.
Lippincott has not abandoned medicine of music as Keystone subjects, of course.
Charles A. Madison, BOOK PUBLISHING IN AMERICA 424 (1966):
[I]n 1958 . . . the firm began to issue Keystone Books, paperback reprints of its backlist in medicine and music.