The Dick Powell book is good, but I don't know if I would still recommend it, with outdated designs and sketches that are just not as crispy as contemporary design sketches. I do highly recommend the Koos Eissen books, those are also used at our design department in Eindhoven. Some years ago, students would take sketching and drawing courses with an actual teacher, now they simply go through the Koos Eissen books by themselves. Though for learning how to draw, feedback from a teacher is still indispensable of course. Koos Eissen also has an older little book called (in Dutch) 'Produkt Presentatietekenen' which may be helpful if you can get your hands on it.
I managed to get my hands on a seemingly rare trilogy of books about Luigi Colani's work that was published in Japan ('Bio-design of Tomorrow', 'Designing Tomorrow', and 'For a Brighter Tomorrow'). You hate it or you love it but it did help me with my drawing process. He advocates very much to start developing form more intuitively, without much reference to rules. I would also recommend Betty Edwards' book 'Drawing on the Right Side of the Brain' just to help you with developing an artistic eye.
And make sure to like this page of our design sketching instructor Martijn van de Wiel! He regularly posts example sketches and student work there.https://www.facebook.com/designsketching