The Massachusetts Historical Society has the best on-line collection of Adams manuscripts in the world. I relied heavily on this site while working on The Midwife's Revolt. Here, you can read John Adams's diaries, and the letters between John and Abigail.
It was a bittersweet experience to go to the Boston Public Library in search of revolutionary broadsides, only to discover that I could see nearly all of them online. Part of me was sad that I did not have to spend hours trawling boxes of dusty, fragile paper. And part of me was quite thrilled that I did not have to spend hours trawling boxes of dusty, fragile paper. I could do my research from the comfort of my own study.
This wonderful website is a glowing avatar of the digital age: read Martha Ballard's diary (1785 - 1812) in an interactive format that allows you several ways to appreciate the original text.