Crowley watched after both of them as they walked past and continued on the mad journey across the book shelves. He took up the rear, just to keep an eye on both of them, to smirk at those red tipped ears, and to spy on that Librarian one last time before the mission carried them on out of viewing range. She had quite the stack of books there, all returns, some of them late, which she was sorting into piles to return to the shelves they belonged on. It her own little world then... maybe she wouldn't have heard the clatter of crystal beads singing off each other like a windchime as Aziraphale nearly whacked that light fixture off the ceiling.

"Angel," this said in a hissed whisper as like radar the Librarian's head snapped up, "Watch your step also means what's hanging in front of you too. You nearly got that lady stirred up again."

He sauntered after the pair, still speaking as he skirted around a stack of books but just missed seeing the rounded bookend concealed behind them. This was ever so lightly kicked with the tip of a snakeskin boot and sent rolling across the stacks. It teetered once on the very edge before gravity forced it right over and sent it to smash into a million pieces on the floor. Glass shards... everywhere...

All Crowley could do was gap at the mess.

"Think she heard that?"