Drawabox. Perspective is the fundamental you should learn the most, it helps with literally everything and is instrumental in solid draftsmanship, you can use in any style, any aesthetic, any subject and it will only improve.It unlocks the simple 2D paper in to 3D space that can be fully filled with anything you want, in a way is like "unlocking the camera" of your artwork, giving you much more choice and dynamic composition in anything you do, plus giving a complete full understanding of shape and form, needless to sya how beneficial that would be to any illustration.

Drawabox teaches you how to fundamentally use this tool in other to understand drawing in a deeper level that can be only achieved trough study.is simple, intuitive, effective and it provides exercises that I still do this day to polish some of my skills and is something an beginner as an experienced artist can benefit. is free, easy to find and I'm always shilling to anyone who asks me how to draw.

You can study anything perspective related and get good results, but draw a box teaches you how to internalize this sorta of thing without relying so much on guide lines and alikes.