What's a good source to learn more about KPI's (product marketing)?

Search in www.google.com

the good source to learn more about KPI you have to understand or focus the customers needs and wants.

Please check ASQ website

There are plenty of resources and training material in the web. One is the best is here: http://www.pnmsoft.com/resources/bpm-tutorial/key-performance-indicators/

It all depends on the business you are in - what is good in one industry, is not relevant in another.

To make the long stroy short - you better keep a close eyes on your Customers, for they are the source of (almost) every important information.

See what metrics are relevant to your business or industry. For example for Pepsi, number of facings in coolers, amount of empty cooler space, and visits per shop were all relevant KPIs. For an accounting firm those would be useless, as would ledgers be to a route salesman. Go to your market and ask, don't waste time on internet searches.