An important reminder: As a general rule, it's best to start your research by consulting secondary sources. Starting with primary sources often forces you to reinvent the wheel when other people have already done much of the interpretative work for you. There are many regularly updated resources available to you when you research Massachusetts law, so don't forget to take advantage of them along the way.
While not techically an encyclopedia, the library has a multi-volume set of essays on MA law called the Massachusetts Practice Series. Written with the student and practitioner in mind, the volumes of this set contains detailed introductory essays on particular areas of MA law. It is an excellent place to start your research. Available online and in print, it has individual volumes to each major area of law. Some examples include:
These guides are excellent sources for a practioner, giving a pragmatic overview of state law in a particular field.
Search the library's catalog (and limit to "Law Library") for print versions on a particular topic. For a complete online listing, visit WestlawNext and type "Massachusetts Continuing Legal Education" into the main search bar.
The Massachusetts Trial Court Law Libraries have a website with a wealth of information on Massachusetts state law, including comphrensive collections of state law in particular subject areas (e.g., criminal law, criminal records, cyberbullying). Membership grants you access to a host of other print and online resources through their system as well.
Note: Massachusetts does not have an official Rules of Evidence; it relies on common law instead, so treatises should be consulted. However, the Supreme Judicial Court Advisory Committee has published a Massachusetts Guide to Evidence and made it available online. (This publication though only offers a guide, not official rules.)