Although I can't comment or give information on the Middlesex Reentry / recidivism rate of the Middlesex County, I can offer a web site related to the States Reenrty / Recidivism rate at:
http://www.mass.gov/portal/site/massgovportal/menuitem.a315d58794c731c14db4a11030468a0c/?pageID=eopssubtopic&L=4&L0=Home&L1=Law+Enforcement+%26+Criminal+Justice&L2=Prisons&L3=Reentry+%26+Reintegration&sid=Eeops
This site is a little off your topic, but I hope it will help you with other information you might use in your research. To find information at a county level will be hard, I assume you tried all links to the Middlesex County and what they provide on their site, if they have one, don't know. There is a Sheriffs related link on this great site related to topics concerning the Sheriffs Departments State wide that might be able to help you. Not sure if you tried it or not.
Good luck on your research aem783. When you complete your research paper and have past, please post it here. Any and all information related to corrections is always appreciated on this site.