I consider David Skok's article on SaaS metrics[1] to be the best articulated and actionable source on this topic. It contains definitions of metrics, why they're important and even an Excel template that I've adapted to run my own SaaS business on. His article on the impact of churn on SaaS businesses[2] is also a must read even if you think you have a handle on it.

Another great source is Christoph Janz's blog, in particular his KPI dashboard for early-stage SaaS startups[3].

[1] http://www.forentrepreneurs.com/saas-metrics-2/

[2] http://www.forentrepreneurs.com/why-churn-is-critical-in-saa...

[3] http://christophjanz.blogspot.co.uk/2013/04/a-kpi-dashboard-...