Reddit reviews Sketching from the Imagination: An Insight into Creative Drawing

Have you gone through all the fundamentals first? Form, perspective, gesture, light/shadow, composition etc? The way im approaching it is to tackle all the fundamentals and slowly integrate drawing from imagination. Im still really grinding through forms right now. Theres a cool tutorial here that talks about how to take form knowledge to the next level and use your imagination to recreate the forms in the way you want vs. what the reference shows.

I think theres another level to it as well that i have gotten from reading a lot of the 3d publishing books, Drawing from Imagination, there are multiple different ones on different categories. None of them tell you exactyl how to do it, more so they show how different illustrators approach the problem. Things that they do include lots of thumb-nailing ideas and keeping "idea" sketchbooks where they can quickly toss down ideas in words or sloppy drawings. I do feel like form and perspective is the basis to all drawing from the imagination though. Without knowing how forms work and intersect with one another or how light bounces off forms, i think it would be a real challenge to ever draw from the imagination. Big shoutout to /r/ArtFundamentals to learn the basics of form and perspective. I think it also takes a long time. Not going to be doing amazing drawings right away, you have to fail a lot.

I don't have any books exclusively like your friend's style, but Sketching From The Imagination is a phenomenal collection of basic sketches from dozens of artists. Lots of different styles in there.