The narrator is struck by Roderick’s excessive nervous agitation. However, he had been prepared for something like this. What had prepared him?

The narrator had been prepared for Usher's condition because of the letter he'd received memories of what Usher had been like as a child.

For something of this nature I had indeed been prepared, no less by his letter, than by reminiscences of certain boyish traits, and by conclusions deduced from his peculiar physical conformation and temperament.