The American Dictionary of the English Language, by Noah Webster 1828, is largely regarded as the pivotal dictionary of Early America. Mrs. Eddy was known to use an edition of this dictionary. Also, it is quite interesting to find that the actual definitions of words have changed over 150+ years. When one is studying Mary Baker Eddy's writings it is useful to use the dictionary prevalent during her time. As such, this can be invaluable. This is a new copy. It is very large and very heavy.
Even harder to find, and now out of print, is the Student's Reference Dictionary, which is based upon Noah Webster's 1828 American Dictionary of the English Language. Fortunate is the individual who can find one of these in good shape today.