The Codebreakers: The Story of Secret Writing, Kahn. Non-fiction. This is one of the standards of the field, and it touches on a surprising array of subjects, including the Rosetta Stone (which was, after all, a code-breaking challenge). Even if you’re not of the class of geek that must read this, it does have plenty of goodness, but do mind the caveats after the jump.

While it is a must-read, it is also a bit of a slog in spots. I understand there was an abridged version issued some years ago, and I can empathize with the impulse, though I also understand that they excised the details of the codes and the methods for breaking them, which is not what slows this thing down. It is, rather, the org charts, to say nothing of the biographical details of virtually every person introduced that bring things to a crawl. If the details provided insight into why the people went into the field, or committed their treason, it would make sense; but it just seems like Kahn, in the spirit of Christopher Tolkien, doesn’t want any scrap of material to go to waste.
When he’s not boring the reader to tears, Kahn provides some brain grist; for instance:

The objective is self-preservation. This is the first law of life, as imperative for a body politic as for an individual organism. And if biological evolution demonstrates anything, it is that intelligence best secures that goal.

I could not help contrasting this sentiment with the Bruce Sterling story that asserts (somewhat convincingly, to me) that intelligence is generally not a survival trait (and, really, check out the cockroach and the shark; brains of a thermostat, and they’re surviving just fine).
I found myself much more strongly in agreement here:

A man can always sustain his convictions in the face of apparently hostile evidence if he is prepared to make the necessary ad hoc assumptions. But although any particular instance in which a cherished hypothesis appears to be refuted can always be explained away, there must still remain the possibility that that hypothesis will ultimately be abandoned. Otherwise it is not a genuine hypothesis. For a proposition whose validity we are resolved to maintain in the face of any experience is not a hypothesis at all, but a definition.

That would be the difference between science and faith.
And speaking of faith, I enjoyed this aside into the evolution of scripture:

The decipherment of cunieform showed that what the West had regarded for centuries as God-given truths had come merely from the human minds of a pagan civilization and, by undermining the divine authority of the moral law, helped pave the way for the ethical and philosophical revolution of today.

I think that’s the inherent danger of requiring some sort of Divine enforcement as the heart of your morality: if the only reason you can think of to behave is that Santa is watching you all the time, once you get the idea that Santa might not know when you’re awake, the behavior is liable to slip some. Despite the song’s exhortation, being good for goodness’s sake doesn’t seem to be an option.