The Restoration of Rights Project has compiled a 50-state comparison on pardon policy and practice. This four section resource provides charts comparing pardon policy, the frequency of pardon grants, how the administration of power is structured and state-by-state summaries including links to more detailed analysis. The section on Massachusetts is comprehensive, and up-to-date. This resource is also useful in determining which state has issued the most pardons, which is an equally important area of study.
The Criminal Justice Policy Foundation has also put together a State Clemency Guide providing information on clemency laws in all states.
New York University Law's Center on the Administration of Criminal Law has been active since 2014 in trying to increase federal pardons as well as study why the frequency of clemency in certain states (such as Massachusetts) has declined. There are a number of articles on various states to pursue on their Clemency Reform page.
Note: There are several articles listed on the bottom of the Clemency Reform page which could be useful. One in particular is The Mercy Lottery: A Review of the Obama Administration’s Clemency Initiative. Included in the article are profiles of individuals who were not granted a pardon by President Obama yet appeared to meet the criteria outlined in the Clemency Initiative.