There's also something, I think, in the fact that Aziraphale keeps bringing people into the bookshop in what is, ultimately, an attempt to tie Crowley more firmly to it. He knows that Crowley's compassion will take over--

Fear not: you're perfectly safe.

(Books are like people, only portable.)

It's much the same as how he knows they cannot run away to the stars, but on a more intimate scale.