Hey! Overall, I would recommend studying anatomy, mainly how to construct the body using simple forms. Drawing the body with simple forms helps you work out perspective and proportions. On the other hand, studying the muscles of the body will help you draw from imagination. One book I always recommend is Michael Hampton's book. The way he draws and simplifies muscles helps you understand anatomy much better. This will help you understand forms better and give the body more volume. It's also crucial to understand how muscles and body forms overlap to create a better sense of volume. Anyway, I hope this drawing helps! :)