For understanding and calculating MRR, CAC, LTV, and cohort analysis, "SaaS Metrics 2.0" by David Skok (of Matrix Partners) [0] is highly referenced. Skok discusses viability of SaaS businesses (such as that LTV should be at least 3x CAC). A list of definitions is provided [1].

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