A rather famous epistolary novel is Pierre Choderlos de Laclos'
Les Liaisons Dangereuses aka
Dangerous Liaisons, which was the basis for the film with Glenn Close and John Malkovitch. It's an older classic and the language in most of the available translations may be too stilted for your tastes, though. But you can read many of them free from the public domain, if you're interested.
One I've personally read is Barbara Hambly's US Civil War novel
Homeland, which takes the form of correspondence between two women friends who are separated between North and South. I personally enjoyed it, but I also think it's the weakest of her pure historical novels (not that this makes it a bad book, as one of her other works was shortlisted for the Michael Shaara Award for Excellence in a Civil War Novel ). It's rather pricey as an e-book, so perhaps you might want to see if your library has it.
Another one I've read is Seth Grahame-Smith's
Abraham Lincoln, Vampire Hunter if you like comedy horror spoofs. This purports to be based on a hidden manuscript diary by Lincoln himself, and intersperses entries from the "diary" along with faux-newspaper clippings and explanatory narrative. It was a moderately fun read which I was happy enough to get from the library, but not one I would have bought myself either before or after.
And if you're interested in science fiction Daniel Keyes Moran's
Flowers For Algernon is a classic in the field, told in the form of journal entries by a mentally-disabled janitor who gains super-intelligence as the result of an experiment. It's been so long since I read it that I don't really remember how it goes, but it's very good and I seem to recall the author uses the language structure and vocabulary choices to really show how the character's perceptions are changing over time.
Also, Wikipedia has a nice list of both classic and modern epistolary novels in
their entry on such, which you may want to consult if you haven't already.
Off their list, I've also read Steven Brust & Emma Bull's
Freedom and Necessity which involves correspondence between a circle of would-be social revolutionaries at the time of Marx and Engels. It was well-enough-written and interesting (I own and enjoy other works by Brust and Bull), but it really is rather involved in history and it helps if you like seriously historical novels.
And I read a few of those
Adrian Mole books by Sue Townsend some years ago and I remember them being mildly entertaining in a spoofily humourous fashion.
Hope this helps.