Drawing and sketching is one of the most basic forms of communication for the Product Designer and it is an important skill to practice and refine. The following texts provide excellent examples of sketch technique and process.
"Sketching" by Koos Eissen and Roselien Steur, is an incredibly broad and practical survey of sketching techniques for product designers. This manual is an asset for anyone interested in good sketch reference and technique.(amazon.com $32.34)
"Innovative Product Design Practice" by Carl Liu, gives great insight into design process, and showcases a plethora of sketches, from rough concept to presentation. The projects present a visual evolution of his ideas, and show how the ideas are refined into more logical and feasible solutions.