Not surprisingly American editions of the paperbacks are used to illustrate the book. There is a checklist of 334 titles (1929 - 1958) at the end of the book, of which 13 had early Canadian paperback editions. Here are two.
Howard Hunt's Maelstrom was first published in 1948 (New York, Farrar, Straus). The Harlequin edition beat the first (Signet 768 - 1950, retitled Dark Encounter) American paperback version to market.
Harlequin's edition of Night and the City (New York, Simon and Schuster, 1946) by Gerald Kersh came out the same year as the first American paperback - Dell 374.
Harlequin 3 - May 1949
Harlequin 3 back
Harlequin 45 - May 1950
Harlequin 45 back