Epistolary novels employ a unique point of view: that is, they're written in the form of letters (or, nowadays, it could be e-mails) between characters.
All the action happens off-stage, as it were, and the reader only hears about it afterwards as it's relayed in the correspondence.
The letter can be to the person involved in the action, or to a third person.
An example of the first might be:
Thanks so much for a wonderful time last night. It was so wonderful to see you - I was so excited waiting for you! How clever of you to think of bringing me to the ballet, I really enjoyed it. And when you kissed me afterwards ... mmm ... 😀
In this case you have to be careful to both give the required information (date, ballet, kiss that she liked), without info-dumping. It wouldn't work to write:
It was wonderful to see you last night. We went to the ballet.
Because, of course he knows he went to the ballet. That's a bit of an extreme example, but the point is that you need to be sure the writer only says something that makes sense, that they would say.
If you have your character writing to a third person it makes it much easier for her to share things that she mightn't necessarily share with the love interest. So:
Well, it finally happened! Gary asked me out. I couldn't believe it - after so long! I was so nervous waiting for him I can't tell you. He brought me to the ballet - I think he thinks I'm far more high-brow than I really am. Wonder what he'll think when he finds out that I'm not really that cultural.
Indeed, as per my examples above - you could have your character write to several people, giving shades of truth to each one, so that the reader gets to know the whole truth.
The letters don't have to be from only one person. This way we can get into the point of view of many characters. So, in our example, Gary might send a quick mail to somebody:
Well, I finally did it! You'd've been proud of me. I asked her out - and she said yes! I brought her to the ballet ... she looks like a cultured kinda girl. I dunno if I can do that again, though - I was so BORED! I wonder would she be on for a football match next time?
I think the difficulty with this style is that the reader can never ever witness any action directly, and no matter how well it's related in the letter, it's still second-hand. This can put a distance between the reader and the action, which isn't good. It makes it harder for the reader to stay engaged in the story, so you as the writer have to work that much harder to make sure that happens.
We tend to think of epistolary novels as being quite old-fashioned. But one of the most popular and modern of writers, Cecelia Ahern, uses this method in her novel Love, Rosie (Rainbow's End), albeit with e-mail rather than letters. And Lional Shriver's award-winning novel, We Need To Talk About Kevin was written in this format too.
If you do use this style, make sure to vary the style of writing for each character's letters.