In the early days of novel writing, the epistolary novel was the pre-eminent form to use. Great works of fiction employed it, like...
Here is Henry Fielding on the subject...
But Mr Fielding wrote that in 1747, and in the 250+ years since then, epistolary fiction has become virtually extinct - although, as with a lot of things in novel writing, just because something is rare doesn't mean that you shouldn't at least consider it.
In epistolary novels, the entire story is told in the form of letters, written from one or more of the characters to other characters - and sometimes even to figures such as God! Their greatest strength is the strong sense of realism that they create.
Nowadays, of course, letters have sadly gone out of fashion - in both the real world and in fiction - but it is perfectly possible to put a modern twist on the concept by substituting letters for e-mails, for example, or even text messages or tweets.
And that is really all there is to say...
Epistolary fiction is told in the form or letters, and the form is virtually extinct!
It is not altogether extinct, but using such a little-used viewpoint would be a brave choice, particularly if it is your first novel.
Why? Because all the usual rules of how to plot a novel, how to create the characters and the like would still apply - but you would need to obey the rules through the medium of letters. Not an easy task.
What is a more likely option than writing an entire novel in this way is to use the form for part of a novel - perhaps the odd chapter here and there devoted to an interchange of letters (or whatever) between characters.
Anybody interested in writing either an entire epistolary novel, or just using the device in parts, should read the classics I mentioned above.
They should also check out one of those rare contemporary examples of the form - one that also happens to be a highly-acclaimed work of fiction in its own right: Alice Walker's The Color Purple.