I'm gonna come right out and throw a vote in for two of the canonical works: <i>Computers and Intractability</i> by Garey and Johnson and Donald Knuth's <i>The Art of Computer Programming</i>.<p>The first one is a great, if occasionally thick, crash course in Theory of Computation and the second is a fantastic series on, well, damn near everything.<p>Huge chunks of <i>The Art of Computer Programming</i> are way beyond me and I've still learned tons from it.