Good Books on Design Sketching
Good sketching skills are important in any design process, and something truly needed in the design industry today. While working in the design industry, I have seen many young designers give up on sketching because they think they cannot do it.
The truth is sketching is a process that requires constant practice to perfect. Therefore the determination to practice is essential to helping you succeed, hopefully to the point where sketching becomes second nature to you. Or to the point where you would not struggle when you have to visualize a design in your mind.
One good way to improve your drawing is to use good sketches and sketch techniques to motivate you. So here are some references that I find useful as a source of motivation.