John Foster (1826). “Essays, in a Series of Letters, on the Following Subjects: On Man's Writing Memoirs of Himself. On Decision of Character. On the Application of the Epithet Romantic. On Some of the Causes by which Evangelical Religion Has Been Rendered Less Acceptable to Persons of Cultivated Taste”, p.67
Robert H. Schuller (1993). “Robert H Schuller: The Inspirational Writings : Tough Times Never Last, But Tough People Do/Tough Minded Faith for Tender Hearted People”, Bbs Publishing Corporation
Peter Drucker, Alan Kantrow, Rick Wartzman, Julia Kirby (2016). “Get the Right Things Done: The Drucker Collection (6 Items)”, p.33, Harvard Business Review Press