D.J. Beaird, New Directions Publishing Corporation, 1936-64 (University of North Carolina 1966).
Henry W. Sommerville, Dissertation, Commerce and Culture in the Career of the Permanent Innovative Press: New Directions, Grove Press, and George Braziller Inc. (University of Rochester 2009), available at https://urresearch.rochester.edu/institutionalPublicationPublicView.action?institutionalItemId=14076
Lots of great stuff on how smaller presses like ND and GP lived off less popular reprints from larger firms like Knopf and Random House. Around p. 267, discussion of GP entering quality paperback market. Chapter on ND starting at 400 goes into a lot of detail. ND and quality paperbacks starting at 425.
Robert Dana, ed., Interview with James Laughlin, in AGAINST THE GRAIN: INTERVIEWS WITH MAVERICK AMERICAN PUBLISHERS, 1-42 (U. Iowa Press 1987).
Records of the New Directions Publishing Corporation, Houghton Library, Harvard University, https://hollisarchives.lib.harvard.edu/repositories/24/resources/1323/.