In a recent, great article in the New York Review of Books, "The Key to Rereading," Tim Parks takes the usual metaphor of reading as "the mind is a key unlocking the meaning of the text" and turns it around:

The mind is not devising a key to decipher the text, it is disposing itself in such a way as to allow the text to become a key that unlocks sensation and “meaning” in the mind.

Parks also asserts that this unlocking process isn't achieved upon a first read, but upon re-reading. Once, again, and again, and again.

The astonishing thing to me at least is that anyone actually needed to say this. Every person who lives in the world of mathematics (and computer science is a branch of mathematics, although if homotopy theory is correct every branch of mathematics is in fact a subset of computer science theory) knows this beyond any shadow of a doubt. I hinted at this earlier is my article "A-hah! Learning and Grind Learning," after which someone said to me that the "A-hah!" moment only comes after the grind, that you've spent all the intellectual energy necessary to push the neurons into the right pattern, after you've felt those neurons reaching out to each other, after the whole of the pattern coheres after hours and hours of study and research and experimentation, there's this sudden "Oh! That's how it works!" moment.

Reading papers, writing code that does what those papers describe, tracing the meaning of that code as it does what it does, all leads to that moment when the key turns in the lock. When you get it. I liken it more to knowledge beating down the doors inside my head, relentlessly creating meaning where once there was none. That's a slightly more violent metaphor than Parks's, but it comes down to the same thing. The knowledge isn't really in either the book or in your head. It's when you've done the same exercise twice and in different ways that the noise of syntax and source code fades into the background, and the glittering beauty of the lambda calculi, or the sheer messiness of ad-hoc continuations like try/catch/finally, become utterly clear.