Epistolary

epis·to·lary
Pronunciation: \i-ˈpis-tə-ˌler-ē, ˌe-pi-ˈstȯ-lə-rē\
Function: adjective

1 : of, relating to, or suitable to a letter
2 : contained in or carried on by letters

3 : written in the form of a series of letters

In an older dictionary it listed epistolary- as a romance carried
out through a series of letters.

"an endless sequence of…epistolary love affairs"
"an epistolary novel"