If you're doing character art, I highly recommend grabbing Artistic Anatomy by Dr. Paul Richer.

It's mainly helpful in being a one-stop-shop for the bones, muscles, and their placements near and around each other. You mainly just get 4 angles of each body part, but it's good mental practice to find the muscles and connect them in your head; helping you imagine a 3D figure while using external searches to verify your thoughts and solidify its appearance.

Also the 250 box challenge. Didn't complete that one, myself, but doing it did help with understanding 3D spacing a lot better for my works

ALSO ALSO, don't worry too much about anatomy in your sketches. Realized shooting for perfection can really mess with the flow of my work. It's better to play with the visuals in a sketch, create a reference, and then combine the stylization of the sketch with the functionality of a reference.

Alright, I'mma get back to work

-Have a sweet night in hell