One of the first books to discuss the differences between male and female sexual desires, and the first book to note that increased sexual desire in women coincides with ovulation and the period right before menstruation. The book argued that marriage should be an equal relationship between partners. In 1935 a survey of American academics said Married Love was one of the 25 most influential books of the previous 50 years, ahead of Relativity by Albert Einstein, Interpretation of Dreams by Sigmund Freud, Mein Kampf by Adolf Hitler and The Economic Consequences of the Peace by John Maynard Keynes. Digital facsimile of the 6th edition (1919) from the Internet Archive at this link. The first American edition was retitled Married love, or love in marriage (1918). Its full text is available from the University of Pennsylvania at this link.
For the fifteenth edition  G. P. Putnam's Sons of London issued a signed, numbered edition limited to 540 copies signed by the author on the frontispiece.