Das konträre Geschlechtsgefühl von Havelock Ellis und J. A. Symonds. Deutsche Original-Ausgabe Besorgt unter Mitwirkung von Dr. Hans Kurella.

This work, translated into English as Sexual inversion in 1897, became the first English language medical textbook on homosexuality. It was published as the second volume of Ellis's Studies in the psychology of sex. Digital facsimile of the 1896 edition from Google Books at this link. Digital facsimile of the 1901 second edition in English, with informative prefaces to both the first and second editions in English, from the Internet Archive at this link.