It is the English version, published by Koestler's cousin Francis Aldor in 1934, of the book L'encyclopédie de la vie sexuelle that Koestler in 1933 wrote, together with "A. Willy" (the pseudonym of his other cousin Willy Aldor) and the German Dr. Ludwig Levy-Lenz.
The second book is Sexual Anomalies and Perversions, Physical and Psychological Development, Diagnoses and Treatment.
[1] This third book was subsequently translated into English and published under various titles and with changes to the structure and text of the original edition.
As noted there, The Encyclopœdia was highly successful and became a best-seller in several languages, but Koestler himself had little benefit of it, being shamelessly exploited by the publisher – a crooked cousin of his.
Koestler received a flat payment of sixty pounds and no royalties from the sales, which amounted to tens of thousands.