a couple of good suggestions already.

I would add:
- Zen or the art of motorcycle maintenance
- Walter Colebatch's The Sibirsky Extreme Project
- Nathan Millward - going postal, the ups and downs of travelling the world on a postie bike (Brilliant! re-published as "Sidney to London - the long ride home" with the addition of images taken during the trip)

I also thoroughly enjoyed Trui Hanoulle - Meisjes Moslims en Motoren ("Girls, Muslims and motorbikes"). Unfortunately only available in Dutch as far as I know.