Luckily it's more of a thump than a crash, but it's understandable that she might be concerned with Cade's sudden start and angry muttering. He's not about to become violent, but with his reputation in Skyhold, it's always a possibility.
He looks at her apologetically when he hears that she's upset, and stoops quickly to pick up the book and offer it to her while he rubs his head with the other hand. "Sorry," he says quickly, "that was stupid."