Eric W.
Yelp
The Mary Baker Eddy Library is worth a look if you are in the area with a bit of extra time on your hands, but it is certainly not a must-see attraction, and I wouldn't necessarily go out of my way to visit. Mary Baker Eddy herself is not a household name to most, but she is well known as the founder of Christian Science, with the publication, Christian Science Monitor still in wide circulation today. I would not say that I am an avid reader by any means, but I am definitely familiar with it.
There is an upstairs area dedicated to the life and times of the building's namesake. It is a great learning experience to die hard nerds like myself (or fans of the magazine), but to be honest, there isn't a whole lot to keep the casual person interested, and children might be apt to signal a run to the exits.
The star attraction of the library is undoubtedly the Mapparium, likely the main reason why most patrons pay a visit. It is basically a giant inverted globe that you walk inside of. Constructed out of 600+ stained glass panes, the map is frozen in time from the world as we knew it in the 1930's. It is interesting to see how names and boundaries of countries have changed so much since.
Visitors are not allowed to enter on their own, so everyone has to wait for the guided tour. Photography is also not allowed. Walking on the bridge through the center of the earth along the equator is a unique and amusing experience. It almost makes you feel a bit like Professor X. Also fun is the fact that the design of the Mapparium itself serves as an echo chamber of sorts. Every whisper, snicker, and jeer can be heard by all. The tour guide initially tells the guests to keep it down while s/he gives the scoop, but afterwards, everyone is free to have at it.
At $6 a pop, the admission price is affordable enough that even if you spend less than an hour, it won't cause too much of a dent. It can add up rather quickly though if you've got little Billy and Sally along for the ride. The Mapparium tour is relatively short, but fortunately, you are able to go back for more rounds if you like.