I love epistolary tales too. Are all the suggestions available as ebooks? Guernsey is available, even as an "enhanced" ebook. (I am on wait list for it now..)
I recently read Meg Cabot's "Boy Meets Girl", a YA romance novel. Light, and easy read. The story is told through emails, journal entries, text messages and even grocery store receipts! I got mine from the local library in epub format. Surprisingly, the formatting was totally off at some places. (I could see the html and css source code!) The print version uses several typefaces, which did not transfer well to the ebook format.
I believe the other two novels in the "Boy meets.." series are epistolary. Bridget Jones books are epistolary.
"Other people's mail" an anthology of short epistolary fiction compiled by Gail Pool, unfortunately not available in ebook, is a very good read too.