Look up "Grays Anatomy", a very well known Human anatomy textbook first published in 1918. I used that book as a visual reference for a project I worked on in 2012. And I own a hard copy of the book (a later edition, of course).
It's got beautiful hand-drawn images of human anatomy like this one:
IIRC it's in the public domain. But I don't know if you're allowed to use it in a book of your own. Please do your research on this matter.
If you can't use images from "Gray's anatomy", then look for "vintage anatomy drawings" or something.