The specialists tell us that Rong Cheng probably was a teacher of the Yellow Emperor (Huangdi) and Laozi and he was an ancient master of esoteric techniques, especially known for sexual practices (fángzhōng shū 房中書), and that is why we have an article in encyclopedia written by Gil Raz: “The fourth of the ten interviews between mythical rulers and sages contained in the Mawandui manuscript, the Shí Wèn 十問 (Ten Questions), is between the Yellow Emperor and Rong Cheng who expounds on breathing techniques and preservation of pneuma (qi).”
The book Róng Chéng Yīndào 容成陰道 (Rong Cheng’s Way of Yin) was lost very long ago and sad things happen in history every day (we know that too), the good news is that men will always try to get equal information from other sources. Usually it helps (Welcome to the Computer Age!). Recently people interested in fángzhōng shū 房中書 reading can find several translations of what have been found in Mawandui manuscripts (Hé Yīn Yáng 合陰陽 (Conjoining Yin and Yang), Shí Wèn 十問 (Ten Questions), and Tiānxià Zhìdào Tán 天下至道談 (Discourse on the Ultimate Way Under Heaven) and much more. Practically, we can choose translation referring to our taste only, from academical to popular and fictional, containing huge commentaries or nothing at all.
For example, you can choose to read Volumes I and II of ENCYCLOPEDIA OF TAOISM for four months of your life or you can read these short notes every time you want pleasure of reflection on ancient aura of philosophy, cosmology and cosmogony, hygiene, sexual guidance, daoyin gymnastics, and longevity.