Armin nearly jumps out of his boots at being addressed, and looks towards Aziraphale dumbly. His mind had already glossed over at the prospect of reading possibly for the rest of his life and never being done, and then he was looking straight into the eyes of an honest to God angel... It was a positively out-of-body experience.

Armin certainly can't see angelic auras on his own, but Aziraphale was still exuding a sort of... Tea and Jaffa Cakes vibe. It was weird. And Armin's never tasted either of those things before.

"T-there are... so many..." he stutters, miming like an idiot at the vast shelves, doing absolutely no favors towards his own actual intelligence.