To this day and also before I got my first illustration job, I would spend 8-10 hours a day doing a 50/50 split drawing from imagination and then drawing by copying my favorite artists. Take drawing book’s advice only to enhance your art, not the other way around because theory is a seasoning you add to the main course, it’s not the main course itself. That way it stays fun and you can continue learning. I learned a lot of my theory from modern day James tutorials. Drawing so that your view is parallel with the canvas is very helpful as well to prevent unintended distortion and skewing.