There are many great works of literature written in the form of an epistolary novel. However, many of those stories could have also been told in a more traditional novel form. What effect does the epistolary novel format have on how novels are interpreted?
For reference, an epistolary novel is a novel written showing various documents that tell the story with little to no other narration.
Epistolary novels should help increase reader identification with the writer-protagonists. The format shifts our sense of the weight of the "invisible hand of the author" from the words being written to the presentation of the individual texts. This helps us put the author out of the way, developing an increased sense of agency for those writer-protagonists.
More agency for characters, plus the diagetic reality of the letters/documents/etc, promotes the reader's sense of verisimilitude. The world and the story gain tangibility.
Further, sustaining that verisimilitude and agency allows the reader advanced interior access to writer-protagonists. Not only can we "see their thoughts," but also we can see them attempting to put their thoughts into a coherent, communicable document. How an epistolizing character constructs their documents should, in a good epistolary novel, show us much about the character.
Epistolary novels are excellent ground for use of unreliable narrators and contradictory perspectives/reports of events, especially if we get more than one character writing letters. We tend to assume, reading an epistolary, that events are being recorded faithfully, that the writer-protagonist is communicating honestly and in good faith, but this can be subverted, requiring readers to revise their perspective and analytic technique.
Epistolary novels also allow for huge amounts of "off-screen" action just by definition. These two points together can generate complex, dynamic plots that belie the seeming-simple nature of the episolary. All this can lead to undermining reader expectations, generating surprise, re-read value, etc.
I could get a whole lot more specific if the question was about the effects of the epistolary form on interpretation of / reaction to a specific novel, but this should at least be a solid partial list to get thinking about the way an author might be shaping your experience of a story by using epistle rather than direct narration. The items I listed should have a fair degree of general applicability. However, a "complete, accurate" answer is pretty impossible here, because conventions and techniques for deploying any stylistic mode change over time, as do the concerns to which an author might apply that mode. That is to say, a Jane Austen novel and a modern CIA thriller are going to use the epistolary form in different ways for different purposes.
