I'm always fascinated when I find really fantastic information about the human figure in completely unexpected place.

Carl Heinrich Stratz was a gynecologist practicing at the end of the nineteenth and beginning of the twentieth century in Germany (though he was born in the Ukraine). He authored several fairly popular and well-regarded books in German on medical and anthropological topics. Although his books are definitely a product of their time and contain overtly racist assumptions that most modern readers would find either comical or offensive, there is also an enormous amount of really great information in his books. Stratz was fascinated with the proportions of the body. Among his corpus of work are many illustrations that break down human proportions based on sex, age, and race/ethnicity

Stratz's books don't appear to have ever been published in any other languages besides German, and his works don't appear to have experienced much popularity outside of Germany. Wikipedia, for example, does not have an English-language page on him, though there is a decent size entry on him on the German version of Wikipedia (english translation).

As a result, finding his books in the English language is practically impossible. But, from what I've read, much of the prose is essentially valueless, filled with outdated science and bunk pseudo-science in the form of "physical anthropology". But, apart from the words, there are literally dozens of illustrations and photographs from his books that have value to figurative artists, including many line drawings showing proportions as well as nude photographs of people of all ages from literally all over the world.

Since Stratz's books were all first published before 1920, they are all in the public domain in virtually every country in the world, meaning his works can freely be copied, republished, or used. Wikimedia Commons has some of his books (Der Körper des Kindes und seine Pflege, Die Fraunkleidung, and Die Körperpflege der Frau), and Google Books has several other of his books available. The Internet Archive also has several of his books digitized.

Here is just a tiny sample of images from his books: