PREP (Prevention and Relationship Enhancement Program) is one of the most comprehensive and well respected divorce-prevention/marriage enhancing programs in the world. PREP is a skills and principles-building curriculum designed to help partners say what they need to say, get to the heart of problems, and increase their connection with each other.
On their website are various resources about marriage, cohabitation, and divorces. It has a bend towards traditional understanding of the family, but valuable nonetheless.
Prep, Inc – Articles. – Of special interest is Marriage in the 90s: A Nationwide Random Phone Survey (PDF).