Boston Public Library, main branch
They're in the circulating library, i.e., the modern wing with the entrance on Boylston St. Go in that entrance, head straight back, go down the stairs (or take one of the elevators) to the basement.
Location
700 Boylston St.
Boston
Handicap accessible?:
Yes
Baby-changing table:
Yes 
Location
Javascript is required to view this map.
Comments
Also some in the McKim Building
From the Dartmouth street doors turn left and follow the signs or go down the stairs where a set are located on either side of the staircase.
BPL restrooms
Please note that due to reductions in the custodial staff the restrooms on the lower level of the McKim Building are usually closed to the general public.