The library's mission is to provide materials and services to help people obtain information to meet their personal, educational and professional needs placing particular emphasis on youth and reference service, and to serve as a repository for local history and art.

The main library has significant historical holdings including the third largest collection worldwide of American Whaling materials, early 19th century Quaker materials, and an extensive genealogy collection. Portuguese language materials are available at the Casa da Saudade branch and Spanish language materials are available at the Howland Green branch.