I tried to find a good book on the mathematics (not the philosophy!) of second-order logic on my usual sources (like mathoverflow.net discussions), but so far they have rendered nothing. Given that, as I understand it, there is some interest on these forums in SOL, can anyone help me with a recommendation? Thanks.

Which edition did you read? The image in the post is of the fifth edition, and some people (eg Peter Smith in his Teach Yourself Logic (§2.7 p24)) claim that the earlier editions by just Boolos and Jeffrey are better.

Cutland's Computability and Mendelson's Introduction to Mathematical Logic between them look like they cover everything in this one, and they are both in MIRI's reading list. What is the advantage of adding Computability and Logic to them? (ie is it easier to start out with, does it cover some of the ground between them that both miss, or is it just good with alternatives?)

The questions on Smoking and Nicotine distinctly lack a middle question "Do you use some kind of smokeless tobacco?" (eg I don't smoke but use snuff almost daily).

Something that kind of interests me; to the Pythagoreans mathematics was magic, involving mystical insight into the ultimate nature of reality. Similar ideas continued in Plato and those he influenced, with echoes down to the early modern rationalists. But Pythagorean mathematics was exceedingly primitive and limited. Modern logic and mathematics are vastly more powerful and sophisticated, and yet the Pythagorean mysticism has almost completely disappeared. Why were people more impressed with mathematics when it was more limited? I imagine novelty was a factor, and they say familiarity breeds contempt, but I have a couple of other hypotheses. First, there seem to be obvious places here and there in modern logic and mathematics where we seem to face choices, where it looks like a matter of "doing this is convenient for this purpose" rather than "this is the only way things could be." This tends to weaken the idea that the fundamental nature of reality is being revealed. Second, there are parts of modern mathematics that are deeply weird (e.g. many things about how infinite cardinals work). I imagine there are people who find it hard to accept that those parts of mathematics at least could be describing any real world.

I'm certainly not arguing for returning to Pythagorean mysticism, but of the two reasons I propose why people might be ruling that out, I'm inclined to think the first reason looks fairly good while the second strikes me as highly suspect. I'm quite curious as to which has been most influential (or whether it's some other factor or factors, perhaps familiarity really is the driving force, or perhaps external influences, say from hostility to mysticism in other fields, are important).

This argument renders virtually everything immoral. Why is having children singled out? Resources spent on a drink from Starbucks are resources that could be spent on famine relief, therefore going to Starbucks is immoral. Resources spent developing philosophical arguments against various activities are resources that could be spent on famine relief, therefore Rachels's work is immoral. And so on.

Understanding the OS to be able to optimize better sounds somewhat useful to a self-improving AI.

Understanding the OS to be able to reason properly about probabilities of hardware/software failure sounds very important to a self-improving AI that does reflection properly. (obviously it needs to understand hardware as well, but you can't understand all the steps between AI and hardware if you don't understand the OS)

Tolerating dreck kills communities. We'd be better off if the bottom 70% of comments were invisible

That's easy to test. Make a private club -- an online community where you have to be invited to join and will be kicked out if you don't measure up. Do you expect such a private club to be a good replacement for LW?

Come to think of it, do you happen to know ANY active, vibrant, useful communities which aggressively expel people not only for being trolls or assholes, but just because they aren't good enough?

Or Quirrelmort is not the Sauron of this story but will help Harry to defeat the main bad guy Death. This could be a really cool ending, but I doubt that it would fit in the remaining arch.

Read Eliezer's short story "The Sword of Good". I half-expect a "The 'good' wizard is only playing the role and really isn't helping make the world be a better place, while the 'evil' wizard is actually the righteous one".

Three shall be Peverell's sons and three their devices by which Death shall be defeated. - chapter 96

The one with the power to vanquish the Dark Lord approaches, born to those who have thrice defied him, born as the seventh month - - chapter 86

There has previously been some speculation that the dark lord in Harry's birth prophesy is death rather than Voldemort. I think this interpretation just got a lot stronger.

James and Lilly had defied Voldemort but not death. The new lines back an interpretation that the Peverells thrice defied death with the three deathly hollows and Harry is born to the Peverell line.

This is, in some ways, a more natural interpretation of that clause since James and Lilly were in the Order and were defying Voldemort on a daily basis not just 3 times. The line of the Peverells makes the number three make sense rather than being arbitrary.

Great idea, but what of the rest of the prophecy ?

And the Dark Lord shall mark him as his equal

That I can't think how to interpret it... how did Death mark Harry his equal ?

But he shall have power the Dark Lord knows not...

That could be any of love, rationality, or hope, the most common hypothesis of what powers Harry have.

either must destroy all but a remnant of the other

The remnant would be memory then ? If death defeats Harry, Harry is dead, but people will still remember him, probably for a long while, and if Harry defeats death, the memory that death existed will stay forever in everyone. Or the remnant of death would be death of non-sentient beings ?