Using your local public library expands your book choices, especially because most can order books on interlibrary loan that will be delivered straight to your home library.
Chelmsford, Groton, Littleton, and Westford belong to the Merrimack Valley Library Consortium. You can search the catalogs of these and all the other member libraries simulatenously here: https://mvlc.ent.sirsi.net/client/en_US/mvlc
Ayer, Shirley, and Townsend belong to the C/W Mars consortium. Their catalogs, along with those of the other member libraries, can be searched simultaneously here: http://catalog.cwmars.org/eg/opac/home