The [author] argues that there are "pernicious effects that inequality has on societies: eroding trust, increasing anxiety and illness, (and) encouraging excessive consumption". It claims that for each of eleven different health and social problems: physical health, mental health, drug abuse, education, imprisonment, obesity, social mobility, trust and community life, violence, teenage pregnancies, and child well-being, outcomes are significantly worse in more unequal countries, whether rich or poor. The book contains graphs that are available online.
There are a number of reasons why our society has gotten more and more unequal. This link will take you to a report that discusses them and then explains what we must do to rewire our economic system for fairness. You can choose the 2-page overview, executive summary or whole report.