What is a good book to learn about business process design?

"Business Process Management" by Howard Smith and Peter Fingar, published by The Third Wave, MK Press. (from2003)

Another one is "Process Management: a guide for the design of business processes", by Jarg Becker, Martin Kugeler, Michael Rosemann (2003). This one is geared towards a process-focused company, so if you're intending to transform a department/company into one that is built around its processes, this might be more relevant for your situation.