I have two book recommendations for you. One is mostly for learning constructive anatomy the other is for perception off shapes and reference.drawing.
The books are called " complete guide to drawing from life" by bridgeman and the other books title is " drawing with the right side of the brain" The books are helpful but remember to practice from references and life as often as you can
There is one thing you will.encounter when you try to.improve in art. It's like an inner voice that will be over critical with your artwork and try to talk you down. If you let it do that it will make you stagnate with your progress in art. You need to stay positive and believe that anything is possible.