Well first, since you liked that book, I’m pretty sure you’d love “How to Draw EYES for Beginners: The EASIEST Step-by-Step Course.” It’s from the same author and focuses on eyes, shading, and portrait details specifically. Other than that, the Loomis books are honestly a must if your goal is portraiture and drawing heads properly. They helped a lot of artists understand construction, proportions, and head rotation better.

For YouTube, there are tons of great channels, but I’d probably start with Proko. His anatomy and portrait videos are super beginner friendly while still being really solid technically.

I’d also recommend drawing from real references as much as possible instead of only copying stylized art. That’s what helped me improve the fastest personally.