Below are some of the the websites and blogs that I have found to be particularly useful in building an understanding of Key Performance Indicators (KPIs) and how to apply them to web analytics, particularly for non-profits.
Avinash Kaushik writes extensively about web analytics. He has three books out on the subject, and maintains a blog with very rich, informative posts that always emphasize the concept of ‘actionable insights’.
Particular posts pertaining to metrics:
http://kaushik.net/avinash/best-web-metrics-kpis-small-medium-large-business/
http://www.kaushik.net/avinash/rules-choosing-web-analytics-key-performance-indicators/
http://kaushik.net/avinash/measure-choose-smarter-kpis-incentives/
KISS Metrics offers a product for measuring web traffic, but they also offer a free and often interesting blog on a variety of related topics. This page has articles specifically on analytics/metrics:
http://blog.kissmetrics.com/topics/analytics/
Shabbir Safdar is a Bay Area consultant who specializes in working with non-profits, and who has a great reputation. He blogs about a variety of issues pertinent to non-profit web work:
http://www.safdaranalytics.com/
Jeff Shuck, of Event360 is incredibly passionate about data analysis. If you ever get the chance to see him in person, do it. In the meantime, he sometimes blog about data:
http://www.yourpartmatters.org/blog/2013/3/21/lies-damned-lies-and-huge-excel-reports
M+R Strategic Services is also a consulting vendor. They’re noteworthy because they produce a benchmark report every year reflecting a variety of metrics across the non-profit spectrum: