In the words of scholars John Bryant and Haskell S. Springer, "Moby-Dick is a classic because it defies classification". It is "both drama and meditation: it is a tragedy and comedy, a stage play and a prose poem", they say, and add that it is "essay, myth, and encyclopedia". The structure is accordingly complex, comprising both narrative and non-narrative elements. Melville's skillful handling of chapters in Moby-Dick, says Warner Berthoff, is a measure of his "manner of mastery as a writer."
Lawrence Buell observes that the "narrative architecture" is an "idiosyncratic variant of the bi-polar observer/hero narrative", that is, the novel is structured around the two main characters, Ahab and Ishmael, who are intertwined and contrasted with each other, with Ishmael the observer and narrator. As the story of Ishmael, remarks Robert Milder, it is a "narrative of education".
The narrative opens with one of the most well-known sentences in Western literature, "Call me Ishmael", seeming to signal that Ishmael will be the central actor, and he is soon joined by Queequeg. But after the Pequod sets sail, the story of these two shipmates is "upstaged", says Buell, by Ahab's monomaniacal quest. Bryant and Springer go on to show how the book is structured around the two consciousnesses of Ahab and Ishmael, with Ahab as a force of linearity and Ishmael a force of digression. While both have an angry sense of being orphaned, they try to come to terms with this hole in their beings in different ways: Ahab with violence, Ishmael with meditation. And while the plot in Moby-Dick may be driven by Ahab's anger, Ishmael's desire to get a hold of the "ungraspable" accounts for the novel's lyricism. Buell sees a double quest in the book: Ahab's is to hunt Moby Dick, Ishmael's is "to understand what to make of both whale and hunt".
The arrangement of the non-narrative chapters, Buell explains, is structured around three patterns: first, the nine meetings of the Pequod with ships that have encountered Moby Dick. Each has been more and more severely damaged, foreshadowing the Pequod 's own fate. Second, the increasingly impressive encounters with whales. In the early encounters, the whaleboats hardly make contact; later there are false alarms and routine chases; finally, the massive assembling of whales at the edges of the China Sea in "The Grand Armada". A typhoon near Japan sets the stage for Ahab's confrontation with Moby Dick. The third pattern is the cetological documentation, so lavish that it can be divided into two subpatterns. These chapters start with the ancient history of whaling and a bibliographical classification of whales, getting closer with second-hand stories of the evil of whales in general and of Moby Dick in particular, a chronologically ordered commentary on pictures of whales. The climax to this section is chapter 57, "Of whales in paint etc.", which begins with the humble (a beggar in London) and ends with the sublime (the constellation Cetus). The next chapter ("Brit") and thus the other half of this pattern begins with the book's first description of live whales, and next the anatomy of the sperm whale is studied, more or less from front to rear and from outer to inner parts, all the way down to the skeleton. Two concluding chapters set forth the whale's evolution as a species and claim its eternal nature.
One of the most distinctive features of the book is the variety of genres. Bezanson mentions sermons, dreams, travel account, autobiography, Elizabethan plays, epic poetry. He calls Ishmael's explanatory footnotes to establish the documentary genre "a Nabokovian touch". Some scholars have tried to identify a single basic genre. Charles Olson saw the Elizabethan play as a likely model, but when he tried to divide the book into acts he found that the chapters resisted this arrangement. F. O. Matthiessen joined in this enterprise, only to admit that some two hundred central pages delay the forward movement of the drama. Northrop Frye found the book to be the best illustration of the "romance-anatomy", but Bezanson cautions us not to forget that the book's "deepest anxieties" stem not from whales but from the Bible and Shakespeare. Newton Arvin tried to link the book to the heroic poem or epic, but found that the book does not fit into epic form.