English translation: Ideal Marriage: Its Physiology and Technique. (1930). The first printing had an insert: "The sale of this book is strictly limited to members of the medical profession, Psychoanalysts, Scholars, and to such adults as may have a definite position in the field of Physiological, Psychological, or Social Research." In its different editions and translations over the nearly 100 years since it was published, it is possible that this work sold more copies than virtually any other book in this bibliography.

Includes a series of 12 "cinematographic" films on 480 flicker cards. In all the photographs frontal nudity of the female model is covered by clothing, though her buttocks are exposed in some views.

Having previously published a trilogy, Ideal marriage,  Sex hostility in marriage, and Fertility in marriage, Van de Velde explained in his preface that "the work to which [his] life is dedicated" was "the spread of knowledge and insight into the sphere of conjugal relationships and the increase of human happiness thereby." Through Sex efficiency Van de Velde intended to teach women a complete system of pelvic gymnastics that would assist them to take "appropriately active participation in the act of sexual congress and appropriate voluntary muscular action which assists the act of birth." When I checked in 2021 I found no record that this book was published in the United States.

Digital facsimile from wellcomecollection.org at this link.