As part of a grand experiment in digital and print-on-demand publishing, I created a volume of these concepts sketches. The process was quick, easy, and best of all free. You can find the various versions of the book here:
Physical Book available via LuLu.com for $24.:
http://www.lulu.com/product/paperback/sketchbook-concepts-from-the-virtual-world/10262884?productTrackingContext=search_results/search_shelf/center/2
Available as a PDF document for $5.95:
http://www.lulu.com/product/ebook/sketchbook-from-the-virtual-world/10269866?productTrackingContext=search_results/search_shelf/center/1
Digital version of the book available via Amazon.com as a Kindle book, which actually looks quite lovely on both the iPhone and iPad for $5.95
http://www.amazon.com/SKETCHBOOK-Concepts-Virtual-World/dp/B001AJNEDA/ref=sr_1_2?ie=UTF8&s=digital-text&qid=1212258757&sr=1-2