Here are all 14 of Joseph John Dymond's Letters on Gospel Ministry, plus the letter from the editor of "The Friend" in response to them.

Letter 1
Letter 2
Letter 3
Letter 4
Letter 5
Letter 6
Letter 7
Letter 8
Letter 9
Letter 10
Letter 11
Letter 12
Letter 13
Letter from the Editor of The Friend

There is a PDF version of all of these letters here.