Compromise
Aziraphale wouldn't say his books were his children but he was wrong. They absolutely were his children and because of this, they were massively precious to him. Many of his books were collector's items – some were hundreds of years old for heaven's sake!
Therefore, he was careful with them. Followed all the correct protocol, wore the cotton gloves recommended, miracled his hot chocolate so there was no risk of it tipping over and staining the pages.
Crowley on the other hand, had no such sensibility. He didn't even wash his hands before picking them up. Aziraphale shuddered at the mere memory. And he left them open, which damaged the spines.
Crowley didn't understand at first. He thought they were just books. Aziraphale was gentle with him. "It's like your plants," he said. "You wouldn't be pleased if someone went around pulling off their flowers."
"I would be furious. I would track them down and rip of their hea –"
"You see? It's the same thing."
There was a pause. "What do I need to do? I don't want to wear the gloves. They make my scales itch."
"You don't have scales my dear."
"Well no. Not in my human form, but I can still feel it under my skin. They make gloves uncomfortable to wear."
"Could you wash your hands before touching my books? It's just that they're special to me."
"I understand angel," said Crowley, resting a hand on Aziraphale's shoulder. "By the way, did you get that new first edition of Dante's Inferno you were talking about?"
"The dealer delivered it earlier this morning. Would you like to flick through it with me?"
"Hell yeah. Humans are so creative when they write about hell. I wish it were nearly as cool as they think it is," laughed Crowley. "Just cubicles as far as the eye can see. But I'll go wash my hands first."