Many have said it before me, and allow me to say it again: this is the Calculus book. Yes, it really does start you off with (a + b) + c = a + (b + c), and then takes you through a nice variety of topics on a breathtaking journey through epsilon-delta proofs, axiomatic deductions and then inductions, the irrationality of pi, the transcendental nature of e, all sorts of sequences and series, and of course--the main 'body' of calculus that is the practice of integration and differentiation. It holds your hand at the sidewalk, and then casts you into the traffic while crossing the road. It is the very paragon of both coldly beautiful terseness and the warmth that accompanies a clear stream of thought moving rationally from one idea to another. It is conversational, yet sufficiently rigorous; it is formidable in scope yet it travels in steps. But each step is a leap--and don't be surprised if you find yourself poring over a paragraph for hours, before the full meaning sinks in. It is Hemingway wedded with Mathematics--the grand iceberg of thought that is analysis lends weight and a grace of movement to the crystal clear tip of Calculus as it glides through the waters of mathematical thought...
As is visible, I quite love this book. It was my first (real) (no pun intended) introduction to Calculus, and oh was it worth it. I must warn you: this isn't for the pregnant and faint-hearted. Or engineers. But seriously--if you're looking for practical application or pragmatic knowledge then this book isn't exactly for you. If you like--or think you might like--the mathematician's ever-refining pursuit for rigour, for watertight argument, for abstract thought--then you'll love this book. Many say it is second or third year level. I feel, for a serious mathematician, it is first-year level at best, though a reasonably bright and motivated high school student can easily get through the chapters, if not solve the exercises to utmost completion.
Be prepared to use your brain, though. I have to make this disclaimer because sadly, in today's day and age, it is almost too much to ask of someone. It seems to have become almost unfashionable, an unnecessary expenditure of effort. But if you are looking to learn mathematics--in this case, calculus--then get your ass over this book and start engaging the grey matter. It's rewarding.