Say what you want about him, but I think Shadwell's absolutely atrocious demon recognizing skills are probably the highlight of his character.
Like, man literally tracked down the demonic colleague of his dead prison buddy, took one look at him, and immediately decided he would do whatever he wanted of him, and for some reason assumed he was a regular human despite everything.
Then 50-ish years later, he sees what looks like a man with an odd light show in his bookshop and immediately thinks, "Ah yes, a demon who seduces women to do his bidding. I must do something about this," with literally no clues to point toward any of that which he was accusing him of.
That's peak irony right there.