Two texts important to the history of Mendon are now available on preservationmendon.org, thanks to Google. Links are at the bottom of this article.
The first, Annals of the Town of Mendon from 1659 to 1880 by John G. Metcalf, M. D. documents the history of Mendon up to 1880. The second Vital Records of Mendon Massachusetts to the year 1850 by Thomas W. Baldwin documents births, marriages and deaths in Mendon to the year 1850.
Google also has a searchable version of Vital Records
Dick Grady explained the motivation for Annals:
In 1876, America was one hundred years old. In response to this
realization, many towns throughout the country decided that one way to celebrate was to re-cap their history. By around 1880, many towns had a John Metcalf type person who had put in hundreds of hours of research and came up with a written history.
Dr. John Metcalf was the secretary of different town boards and had access to many town records. He had great writing and organizational skills. He was very bright, having graduated from Brown University and Harvard Medical School.