Secondary sources are an excellent way to quickly understand the background of a legal subject area. I have included a few here, but you should check Westlaw for more! The articles included here focus on clemency in Massachusetts.
Willie Horton's Shadow: Clemency in Massachusetts
Beg Your Pardon: A Call For Renewal of Executive Clemency and Accountability In Massachusetts
From Here to Clemency: Navigating the Massachusetts Pardon Process
The legislative history of the Massachusetts General Laws of Pardons by the Governor is also worth pursuing. See M.G.L.A. 127 Section 152. Use the compare versions button in Westlaw to see when changes where made to the law.
The following statutes, regulations and guidelines relate to clemency in Massachusetts:
There are only a few cases on clemency in Massachusetts. The easiest way to access caselaw interpreting the Massachusetts constitutional provision on pardons is to look at Massachusetts General Laws Annotated, available through Westlaw Edge.. Article VIII includes Notes on Decisions that you can look through. I have listed a few of the cases here.
In General
Ladetto v. Commissioner of Correction (1977) 369 N.E.2d 967, 373 Mass. 859
In re Opinion of the Justices (1938) 17 N.E.2d 906, 301 Mass. 615
In re Opinion of the Justices (1912) 98 N.E. 101, 210 Mass. 609
Kennedy's Case (1883) 135 Mass. 48
Initial determination
Juggins v. Executive Council to the Governor (1926) 154 N.E. 72, 257 Mass. 386
Refusal to Pardon
The Massachusetts Trial Court Law Library has produced a compilation of resources on sentencing, probation and parole that includes resources on clemency. See Massachusetts law about sentencing, probation and parole. The Massachusetts Parole Board has also produced an excellent article on the executive clemency process in Massachusetts, which goes into detail about the various steps of the process. These two articles provide a good foundation in understanding the clemency process.