Bibliophile bondage.
"I wasn't finished," said the angel imperiously, and started on the other nipple. Crowley moaned. "As I was saying," Aziraphale said between nips, "the care of books is a delicate one." He moved up suddenly and pulled the demon's head back by the hair. Crowley grunted, suddenly nervous about exposing his neck to an enraged angel, but said angel only looked at his bobbing Adam's apple and continued.
This piece of nerdy genius by an anonymous commenter at the
GO Kink Meme. Although regardless of bibliophile bondage, it does annoy me that GO fandom don't seem to pay attention to the statement that possessing sexual organs requires effort for angelic and demonic beings. Everyone seems to assume that they just have them as a matter of course, possibly because the porn is better that way.