I’m a terrible artist myself, but i have some good material on the subject and can recommend some books/courses:
This one has very little text, but as a reference material, it’s as didatic as it can be. Shows all muscles, multiple angles, where each part starts and ends, all with colored pics and some diagrams
This one is beautiful and has these cool semi transparent pages with bones and muscles you can put on top of a photo of the represented part. But the text is overcomplicated and talks more about artistic sensitivity and such than technical bit.
https://global.oup.com/academic/product/atlas-of-human-anatomy-for-the-artist-9780195030952?cc=us&lang=en&
This one is a classic and is as much about artistic sensitivity as it is about the technical side about how each bone and muscle relates with the surrounding parts. It was written in the fifties, and almost everything on it was hand drawn at the time.
Loads of content and a bit of medical jargon here and there, but if you were to start studying anatomy today and could only choose one source, i’d point to this one.
https://archive.org/search.php?query=creator%3A"Loomis%2C+Andrew"
Andrew loomis is one of these “old masters” constantly referred by other artists. His method isn’t overly complex, and yet is extremely effective AND FUN!
the classic technique below, for example, was popularized by him
While they’re made to teach drawing, many concepts translate really well to 3d sculpting (proportions, dynamic poses, “make your drawing tell a story”, etc.
The content is in a bit of a grey area, though: he died in 59 and his last book was released in 61. They became a bit rare phisically, but the internet archive still keeps copies of the age-old versions and it wasn’t taken down even today. This made much of the content popular for drawing courses and such, even if it isn’t public domain.
Only recently (less than a decade ago) new editions were released in US (most other countries are out of luck).