So, "Unsong" by Scott Alexander. In this alternate history, Apollo 8 never reaches the moon, because it hits and cracks the celestial sphere surrounding Earth.
Turns out the torah and kabbalah are literally correct in this universe.
The crack in the sphere is how lights gets in to Earth: divine light, Names of God giving the speakers power, but also magic, suddenly working. Of course, this being the US, this gets exploited for money-making.
(1/n)
Names are copyrighted, and our protagonist Aaron works for an outfit that tries to find new Names by permutation.
He finds one, and things turn sideways fast, sweeping him and his friends up in an all-encompassing struggle against evil.
Because it also turns out that if divine light gets in through a crack, so can evil forces; the devil is at work on Earth, and so are fallen angels and demon armies.
(2/n)
The worldbuilding is neat and thorough; the book takes its premise seriously and finds best ways to emphasize it by little details. It also is an absolutely WILD ride, full of ideas and puns and kabbalistic connections between anything and everything; a bit like a novel-length tumblr shitpost with a bit of storytelling and not much character development. If you like the "Illuminatus!" trilogy or Eco's "Foucault's Pendulum", this might be for you.
(3/n)
The only thing missing for a true tumblr thing is the horniness. There is pleasantly little of that; one character is asexual, and the sexy robot girlfriend trope is subverted: young dude Aaron gets his, but she isn't his type.
There was much I liked (Uriel!), even though I still dislike the assumption that the world gets less infused by divine light when it runs on a mathematical framework. Thanks again @lizzard for the recommendation!
(4/n, n=4)
Jules