Okay, I just read the bit where Harry was hiding under a veil watching Bock's Ordered Books, he sort of reliving memories, because Cowl, Kumori, and Lash were very much a part of his memories of that area. Then he thought of Lash again and how she died to save him and that was the last time he saw Cowl..
Now the important bit I think;
That brought up the thought that perhaps the Coins of the Knights of the Blackened Denarius were connected to Cowl somehow, and one of my eyes started twitching.
I say maybe in Mirror Mirror, a short story, or some other future book, the above was a real clue. Cowl and the Denarians are very much connected somehow, stay tuned.
What further underscores this, is on the next page the first paragraph says he is mulling over the various possibilities and directions for this in his head before he has to pay attention to the task at hand.