I'm jumping in on the "books that changed my life" meme, hopefully bringing out some different titles but inevitably contributing to the already considerable notching in Marx et al's belt.
So I'll start with something that appears on plenty of lists but not usually amongst the theory crowd: Moby-Dick; or, the whale. If you've picked it up and found it senselessly convoluted and frustrating, I can't fault you. I would say it is a difficult book, not in the sense that only smart people 'get' it, but in the sense that some people are difficult people and whether you like them or not is a matter of taste. But for me it is a major monument on the path toward integrating economic critique with the linguistic turn, and the way in which those cathex around bodies of animals concrete and virtual. That richness is not on the surface of the book, in the way that a particularly stunning argument might be immediately mind-boggling once understood, but like an influential teacher who only affects some, Moby-Dick spoke to me of what I wanted to think about.
An example of that other type would be The German Ideology. I remember reading the "Theses on Feuerbach" and thinking something like, "whoah, I can make different kinds of arguments now." But it actually took me much longer to fully absorb how constraining and liberating historical materialism is for argumentational validity. There is a way to argue from "historical materialism" that is fundamentally idealist, and it was precisely that that initially appealed to me. In working through that phase--and reading The German Ideology closely, particularly concerning its use of animals--I had to change how I thought. It's been my experience that thinking is usually very resistant to change, and I feel fortunate to have experienced what I can only call a conversion experience.
Nietzsche would be another slow-acting conversion experience. I read Zarathustra after high school, Genealogy of Morals as a freshman, and so on. Each time changed me a little. I re-read The Gay Science recently and it added new kinks to arguments I'd been working through. Nietzsche has been not so much an event in my intellectual history as the on-going event of my intellectual history.
Deleuze on S&M and the works of Flannery O'Connor I'd put together in developing my understanding of thought as embodied. The Deleuze is probably familiar, the O'Connor maybe less so. Her writing is sadistic but religious rather than sexual. While I don't share O'Connor's Catholic vocation, I acknowledge the importance of religious structures in non-religious modalities of life and find O'Connor's treatment uniquely revealing of the sensual character of the transcendent. She's a great stylist and reliably has unusual, sinister metaphors. And I enjoy the suffering of others.
Why do I study animals, or 'do' animal studies? Strangely, books seem to have little to do with that. While I enjoy and value many works in the field my motivations arise from the interest and complexity of life I find in encountering other animals, rather than the intellectual acrobatics that such a project possibilizes. If there's one book I think is of crucial importance to the field that has not been sufficiently worked through, it'd be Specters of Marx. In my view, the animal/s will remain an ideal figment or positivist sacrifice unless thought in motion, and when in motion its form is specter (this mini-argument parallels some of the major moves in Parables for the Virtual while adjusting for some deep-rooted inheritances concerning "the animal" in the development of Western thought). Plus, I just love Derrida.
A picture from Princess Mononoke was the best I could come up with to illustrate that claim.