Definition of an Epistolary Novel (from Wikipedia):
An epistolary novel is a novel written as a series of documents. The usual form is letters, although diary entries,newspaper clippings and other documents are sometimes used. Recently, electronic "documents" such as recordings and radio, blogs, and e-mails have also come into use. The word epistolary is derived from Latin from the Greek word ἐπιστολή epistolē, meaning a letter (see epistle).
The epistolary form can add greater realism to a story, because it mimics the workings of real life. It is thus able to demonstrate differing points of view without recourse to the device of an omniscient narrator.
The challenge, then, will be to read books written in the form of letters, diaries, blogs, e-mails and such, either completely or partially written this way. Here are some links to get some reading ideas:
Goodreads Best Diary Novels
Goodreads YA Diary Novels
List of Fictional Diaries
7 Variations on the Epistolary Novel