One of the best introductions to 18th-century letter writing is that by Frank Baker in Vol. 1 of the edition of John Wesley’s letters. Wesley was a Methodist minister who lived from 1703–1791, and he was a voluminous letter writer. There is a collection of letters from his hand available online called the Letters of John Wesley at Bridwell Library. This is a digital collection which presents 138 letters by Wesley letters, in the form of scans along their transcriptions. According to the website, these transcriptions are fully searchable.

Reference:

Baker, Frank (1980), The Works of John Wesley, Vol. 25, Letters I, 1721-1739, Oxford: Clarendon Press.