Five hundred and nineteen authors wrote for the big three early Canadian paperback publishers - 262 for Harlequin Books, 143 for Wm. Collins Sons & Co. Canada Ltd's White Circle imprint and 114 for Export Publishing Enterprises Ltd's News Stand Library imprint. There a few authors who had books published by two out of the three but only one who had a book published by all three. His name was Thomas Stone.
Who was Stone? Stone was a she - Florence Stonebraker. Quoting from this site:
"For thirty years – between Pay for Your Pleasure (Phoenix, 1937) and Predatory Woman (Beacon, 1967) – [Stonebraker] cranked out more than 80 novels of unsanctioned sex. Married or single, her characters were tempted by and often surrendered to their lustful desires. Stonebraker had a conventional side, too, and wrote a couple dozen stories of chaste young women finding love. But her forte was the risqué."
She also used the pseudonym Florenz Branch. In order of published date here are her five books from the three Canadian publishers.
News Stand Library 21 - January 1949 (first published Phoenix Press, 1946 as A Talent for Love)
News Stand Library 27 - February 1949 (Phoenix Press, 1947)
New Stand Library - September 1950 (Publishers Productions, 1950 - Ecstasy Novel #6)
White Circle CD 484 - 1951 (Phoenix Press, 1943)
Harlequin 142 - November 1951 (Phoenix Press, 1944 as Doctor Tony)