I’m not an expert by any means, so take what I have to say with a grain of salt. But I’ve spent a good amount of time trying to learn this stuff for myself.

Study of anatomy — human anatomy specifically — is something you can spend your whole life doing. I think answering your question first requires knowing what your art goal is (anime/manga style art? Highly rendered portraits? Mechanical design?) so you can figure out how deep your anatomy knowledge needs to be to get rolling on what you actually want to draw for yourself, and then you can slowly study more anatomy for the rest of your drawing career on the side. This is the strategy I’m taking, at least, and others may have different ideas.

That said, Proko offers maybe the biggest, deepest dive into human anatomy on the Internet. The courses cost money but some of the videos are free on YouTube.

The anatomy books by Bridgman can be pretty opaque with some inscrutable drawings, but his philosophy of interlocking forms can really help you better understand how different pieces of the human body fit together in a three-dimensional way that might feel like a good extension of the Drawabox philosophy.

I think Michael Hampton’s Figure Drawing: Design and Invention book gives you a solid baseline understanding of anatomy, but it doesn’t go super deep.

The esteemed drawing instructor Marshall Vandruff has a list of his preferred books on human anatomy here:

https://www.marshallart.com/HOME/reviews/human-anatomy/

Depending on your drawing goals, you can get pretty far just with a basic understanding of the skeleton and the superficial muscles and their function, and you can get that from a mix of YouTube videos, medical anatomy books, and the best of the “how to draw” books, like those by Andrew Loomis or TACO or the Morpho books. Make sure you’re getting a healthy dose of both real anatomical information and artist’s ideas of how to depict that anatomy. Don’t just study how other artists draw the body.