In her famous novel Frankenstein, alluded to above, Mary Shelley employs the epistolary form very effectively as the letters between Walton and his sister provide an objective measure against the narratives of Victor Frankenstein and his creature.  These letters also increase the suspense as Walton tells his sister of a "wretched man" he meets who pursues a "demon." 

Further, with the motif of listening that courses through the narratives of Victor and the creature, there is the added narrative of Walton, whose desires parallel those of Victor, for he, too, pursues unknown territories of nature and science. Thus, the letters, or epistles, provide a further dimension to the novel through the lessons that Walton learns about himself from his retelling of Victor's story. In his last letter to his sister, Walton is humbled and writes,

...the men, unsupported by ideas of glory and honour, can never willingly continue to endure their present hardships.

So, heeding their pleas, Walton agrees to turn back to England for the safety of his crew a wiser man for having listened to Victor and his creature.

The epistolary form, then, underscores the themes, motifs, and veracity of a novel through its use of informative letters.  The verisimilitude of characters is established also through the use of the letters along with the generation of suspense in the reader by this form.

An epistolary novel is one that is told through the literary device of  letters, exchanged between the characters of the novel.  The first epistolary novel was Pamela; or, Virtue Rewarded by Samuel Richardson in 1740, which was basically about Pamela's attempts to escape her lecherous master.  One of the most famous examples of the epistolary novel was The Sorrows of Young Werther by Goethe which became hugely popular in the late 1700s, so much so that Napeoleon Bonaparte even carried a copy of it inside his jacket on his long military campaigns. 

The primary advantage of the epistolary novel is the way in which the reader develops a connection to the characters and events through the device of the letters, making the action and plot of the story seem more real and immediate. 

Other notable novels that use the device of letters include Frankenstein by Mary Shelley and Dracula by Bram Stoker, which coincidentally are both 'monster' stories.  C.S. Lewis adopted the epistolary format for his novella The Screwtape Letters.  More modern versions of the epistolary novel are Carrie by Stephen King, who also incorporated newspaper clippings, and several of Meg Cabot's wildly popular romance novels, like the Princess Diaries series, The Boy Next Door, and Every Boy's Got One, in which the letters take the form of text messages and emails.