What is interesting about the form of Mary Shelley's 'Frankenstein'?

The 2 key formal features of Frankenstein are the use of epistolary form and an embedded or framed narrative.
A novel written in epistolary form develops the narrative through a series of documents- in this case, through letters and diary entries. Frankenstein begins with a series of four correspondences from Robert Walton to his sister. Shelley uses this technique to position Walton as the chief narrator and create her framed narrative. Imagine a painting- there is, on the outside, a frame or border which contains the piece of art. This is exactly how Frankenstein works. At the beginning of the novel, we have Walton's letters; this is the outer frame. Contained within those letters is the description of Victor Frankenstein, and within that narrative is the story of the monster. There are three different narrators of the story. Walton talks about his expedition in the first person- Victor's story is then told through the letters- the monster's narrative is told in turn, through Victor. At the novel's conclusion, we return to the outer frame and the perspective of Walton, who articulates Victor's death. It is a very complex 'Russian doll' structure which illustrates the power dynamics between Shelly's characters. Walton's story is linked through structure to Victor's- this is because Victor represents what Walton will become if he continues to test the boundaries of nature. Victor's narrative is also structurally linked to the monster's- this acts to emphasise the similarities between the creator and his creation who are both, in contrasting ways, monstrous. Finally, it is worth noting the placement of the monster's narrative- in the middle. His story is trapped between, and isolated by, the other two, distancing the creature from the 'humans' of the novel and presenting him as 'Other'.