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Saying that Moby Dick is about whales is like saying the Old Testament is about keeping kosher. Whales are a very tiny amount of a complex whole.
First, it is about obsession. We think first of Ahab's obsession about killing the whale, but careful study of the book shows that there are many obsessions present. Contained within the obsession is that kind of hubris which challenges gods to do their worst.
Second, it is about piety and impiety, about religious belief and sacrilegious beliefs--beliefs plural, because there are idolaters aboard the ship.
But most important, it is about human beings. Everybody is distinguishable from everybody else, unlike many novels in which it is virtually impossible to tell who has what relationship with whom. It is realism of the American, Andrew Jackson, line, not of the European line.
Deconstructionists say that there is at least a hint of homosexuality in the book. They may be right; certainly Queequeg's calling Ishmael his wife is such a suggestion, even though there is no evidence that even Queequeg, much less Ishmael, ever acted upon such a suggestion. However, temporary homosexual activity even among normally heterosexual men is known to be, if not common, certainly not unheard of in any situation in which a group of males are isolated together, without access of any kind to women. A whaling ship, which might not touch land for two or three years, certainly was such an environment.
I cringe when I hear it described as an adventure novel. It is not one, and the abridged editions which remove all of Ishmael's comments which seem extraneous to the book should be burned and replaced with unabridged editions. Those "irrelevancies" are part of the heart and core of the book.
My husband, when at UCLA, was told by friends that Moby Dick was an extremely difficult book, so he decided, for the only time in his life, to buy Cliff Notes. Halfway through the Cliff Notes he decided that Moby Dick was the best novel ever written in any language. He threw away the Cliff Notes and settled down with the book. At the beginning, before the celebrated line "Call me Ishmael," there is a long series of quotations about whales, none of which are really about whales. He is of the opinion that you could remove whales from the book and still have a good novel, but you could not remove Ahab.
Hollywood has made at least two movies about Moby Dick. Both are good movies, but it is clear that the screenwriters did not grok the book.
I recommend this novel not to children, not to undergrads, not even to graduate students unless they are willing and able to take the time to study Moby Dick, using their own contexts as well as the context in which the author was working, to attempt to get a whole on some of the meanings of the text. This assumes that the reader understands that in so complex a novel, and there are few novels more complex, there is not one right meaning. There are multiple meanings which interweave themselves inextricably, while other meanings seem to grow up not from context or subtext but from intertextuality, particularly intertextuality the Bible and specifically the Old Testament.
This is not an easy novel. But it is one worth reading by a reader willing to put in the work necessary to comprehend it in part, realizing that comprehending it in toto is impossible for anyone.