Are there any resources like the book "Drawing on the right side of the brain" or maybe the more apt book "Drawing in the Digital Age" but for Excalidraw?
Wrong, there are some excellent books that can teach drawing skills very well. I'm a trained studio artist and I've found great value from books. I recommend Bridgman's "Constructive Anatomy". Go through the book and copy every drawing. You WILL improve. Even after years of practice, I was able to take my drawing to the next level. There are lots of other great books out there. Drawing isn't rocket science.
So either your question is how to learn to draw using digital as your art medium in which case you are not going to be using excalidraw.
Or you want to learn to draw only diagrams in specifically excalidraw.
Since your resources are more general drawing and not diagramming I just assume you mean drawing.
In which case how you learn to draw digitally starts with learning to draw on paper.
If you are bad with traditional mediums and materials you will be still bad with a stylus or god forbid a mouse in a digital medium.
Look up sketching tutorials on YouTube and start on paper with pencils, pens, and markers.