From the UW Libraries History Research Guide

"...an online museum of correspondence which has been celebrating the humble letter since 2009."

Use the SEARCH bar at the top and search "letters"

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"...the most wide-ranging online collection of edited correspondence of the early modern period, linking people across Europe, the Americas and Asia from the early 17th to the mid-19th century — reconstructing one of the world's great historical “conversations”

"...digital documents relevant to the fields of Law, History, Economics, Politics, Diplomacy and Government. We do not intend to mount only static text but rather to add value to the text by linking to supporting documents expressly referred to in the body of the text."
Search "Letters" in the box in the upper right.

Follow the path Explore >> Open Access >> [Scroll down to red Open Access bar] >> [Scroll down left margin to Object Type, Show More Object Types >> [Scroll UP the list to Correspondence (50). Happy Browsing!