I have had this book over two years now, and would highly recommend it. Design Sketching is a extensive selection of inspiring sketches by 24 students at the Umea Institue of Design. The book has been compiled by Erik Olofsson and Klara Sjolen. Although you can only buy this book from the www.designsketching.com .
On a side note, Erik Olofsson one of the editors has also released a new book. Called Sketching (12th printing): Drawing Techniques for Product Designers . I havn’t got my hands on it yet but it look like these are the sort of books product designers need.