Alekhine's Book on Chess
How Alekhine's book on chess relates to Aziraphale's heroic risk.
It turns out there's a famous "Alekhine Defense" in chess. The idea is that when White makes the first pawn move against us, we, Black, respond with an unexpected move: immediately move a knight (black «dark» horse), thereby exposing the white pawn to attack.
It's believed that Black is taking a huge risk by introducing an expensive piece into the game. However, it makes it difficult for White to defend the center. Black thus "draws fire" on himself.
Aziraphale is chess black knight, our "dark horse", as Nina called him. This means he's the one acting boldly and riskily in fin-15. This means the white pieces are clearly Heaven, led by Metatron. Aziraphale deceived Metatron in fin-15, convincing him of his loyalty, thus gaining a chance to save himself and Crowley. At the same time, Aziraphale is a threat to Heaven, undermining its defenses.
By the way, my favorite number of squares on a chessboard is 8 by 8! 😊 Two eights, two happy infinities, symbolize the eternal love of my favorite characters.
I wrote about the symbolism of numbers in "Good omens" here.