Being an Effective Onsite Customer or Product Owner - great description of the ideal Product Owner.

Agile Product Owner Boot Camp - nice little bit in here about collaboration.

Certified Scrum Product Owner Course - don't know how good this is, but might be useful.

Here is my suggestion for a definition of the Product Owner role:

The Product Owner holds the final authority for determining the value, priority and details of all work done by an Agile team. The Product Owner wields this authority by virtue of a deep knowledge of the goal and end results desired as well as a respected position among all the stakeholders.

The Product Owner role is also known as the Queue Master, the Customer. People who have done the Business Analyst and Product Manager roles are often good candidates to fill this role.