I had a variant of this conversation with a friend yesterday after a class in which the professor claimed that "David Foster Wallace" was the moral position she would like to claim as her own. While we didn't refer back to that remark, we discussed the role of hyperbole in talking about books. For many (though not, or at least very rarely, for me), hyperbole seems to be the only adequate mode for describing one's feelings about a small set of books which one feels must be lifted out from the basic continuum of comparison and evaluation. These are the books which "change one's life."
That locution—"that book changed my life"—has always puzzled me, as its literal meanings seem generally dubious. If I were, however, to assume that a literal meaning is possible, it seems like it could mean all kinds of things: my behavior changed after I read it (I began recycling more, I watched television less, I became a vegetarian); the way I perceived things changed (I became cynical, or religious, or existential); I felt older or more mature or wiser; or (perhaps as a subset of one or more of these) my ideas of what fiction can be or what fiction is changed. I'm not sure the last would count for everybody as a life-changing event, but for "serious" readers, I think it probably does.
Or, more likely, the book happened into our hand at a moment when we were already re-directing our steps slightly, and we conveniently use that coincidence to narrate and make more definite our account of ourselves. "Life-changing" is less a property of the book than of time, of the narrative of maturation, Bildung, disenchantment, etc. The Catcher in the Rye doesn't make you a nihilistic smartass; you just recognized that you were one after you read it, and you remembered it as a cause, not as a coincidence.
I'm quite skeptical of the other possible meanings for "that book changed my life." Feeling older or more mature or wiser because you read a book is probably somewhat valid, but again, I'd argue it's less a causal relationship and more of a correlation: you simply find the reading of a book a convenient milestone for marking your maturation, not an actual producer of more mature feelings or ideas. Books, like poetry, make nothing happen; they confirm what has happened or what we think has happened, in us as much as in the world.
I imagine there are cases of sustained self-induced behavioral modification that have been caused by reading, but the key word there is "sustained." Who doesn't stay away from McDonalds for a few months after reading Fast Food Nation? But if fast food was a solid part of your life before the book, it probably will be again, and if it wasn't—if you already did not eat fast food often, can your new resolution not to eat it at all be said to be life-changing? Similarly, I find it likely that if L'Étranger turned you into an existentialist, then something else will soon turn you into a nihilist, and Brideshead Revisited will probably Catholicize you if you read it when you're old (and not when you're young and just think rich British people of any religious persuasion are awesome). A convert is rarely a convert to just one thing.
Similarly, reactions to a book which could be described (in a not entirely pathologizing manner) as 'obsessive' strike me as unlikely to form without a supporting structure in place. In the linked example, Greg Carlisle describes his experience of reading Infinite Jest as the cause of his writing Elegant Complexity, a guide to Infinite Jest, and to his further reading of Vollmann, Barth, Gaddis, and Pynchon. The hyperbolic narrative Carlisle gives is that this prolonged activity is the consequence of reading Infinite Jest, as if the chance encounter with the novel which he describes contained within it the entire complex of motivations, energy, and even education necessary for the production of his guide and his further reading—it taught him about and impelled him toward all these other things. In fact, we can see in his account the outlines of some deeper motivations (dissatisfaction with his job and his location, positive reinforcement from fellow Wallace fans) that might provide the necessary grounds for sustaining a form of activity of this duration and intensity.
But what about the last possibility: that a book alters what we think of as the bounds of fiction: what it can do or what it is. I think this possibility is more likely, but it may mean something different from what we habitually mean when we say it. Given that few readers—even few serious readers—have explicitly articulated conceptions of what fiction can do and what it is, I think we have to acknowledge that the reaction "this book changed my idea of what fiction can do/is" would be more accurately re-phrased as "I hadn't thought of fiction as connected to that realm of experience." When we say our idea of fiction has been changed, we mean that we find fiction standing suddenly between us and some aspect of the world that we were either unused to interacting with in the first place or were used to interacting with in a manner unmediated by fiction.
Some of the reactions in the Infinite Summer wrap-up post demonstrate this sense pretty well: Avery Edison notes her sudden interest in tennis and her sudden disinterest in caffeine; Eden Kennedy simply says, "I’m certain little connections between the book and my life will continue to click together over time" and flags a particular interaction with her dentist which was mediated by IJ; Kevin Guilfoile (and Avery and Eden as well) speaks to the way IJ has entered his thoughts about time management and attention. (Matthew Baldwin doesn't so much affirm that IJ changed his life as admit that he can understand why people would say that sincerely.) In each case, Infinite Jest has newly stepped between an aspect of experience and the reader, and it is this new experience of mediation that counts as "life-changing."
Yet this mediation effect (which I find the most compelling description of the actual experience of having one's life or concept of fiction changed by a book) is so much less dramatic than the forms of expression it commonly takes that I feel there must be something other than the real intensity of the experience which necessitates hyperbole. That is, if what we actually feel about a "life-changing" book is that it has suddenly come to stand in between us and a realm (or more than one realm) of experience previously unknown or unmediated, this feeling seems insufficiently powerful to justify the expression "life-changing." Hyperbole is employed because it seems like it is the best approximation of the feeling of excess, effusion, effervescence that characterizes our full experience, and I have trouble finding that effervescence in my account.
My speculation is simply that this effervescence is provided by the desire to have one's life changed by a book, the desire to be a person whose life/identity/consciousness might actually be altered in a sustained and concrete way by a totally private experience. The hyperbolic expression "this book changed my life" is little more than a declaration of independence: I can change my life on my own; what I have inside me needs only the right key for its fulfillment. The frequency with which people (like Carlisle) avow the contingency of their encounter with the "life-changing" book is a strategy of minimizing the determinative structures which have brought you into contact with the book at all. Carlisle emphasizes the casualness with which Infinite Jest first came to him by his friend's recommendation, but this hides the fact that most people don't have friends who recommend Infinite Jest to them; being a person who does is not random or insignificant.
I imagine that all of the above may be considered a giant case of "missing the point," and I sympathize with that view to some extent. There is a communicative value to statements like "this book changed my life"—they are meant, I think, as sincere efforts to share something of the self with others; they are offers of a type of exchange which is intended as more than book-chat. The fact that this exchange is meant to reinforce the idea of an autonomous, largely self-forming self may, in the end, be somewhat incidental; so many of our activities, particularly our intellectual activities, are meant to reinforce the very same idea that perhaps it would be best to pass over this one, in the future at least. While I doubt I'll ever express myself in this hyperbolic manner, I certainly won't repeat this lecture if someone else does.