The man had a strange way of regarding one's face, one's clothing, one's boots; and of listening.
As a watchdog, I could appreciate the mode of total attentiveness he assumed. It was not a normal human attitude. It was as if his entire being were concentrated in the moment, sensitive to every scrap of intelligence our encounter furnished.
A fine, concise description of Sherlock Holmes, wouldn't you say? If it is Holmes. (Of course it is.)
Snuff is a watchdog. And more. As he tells us from the beginning:
I like being a watchdog better than what I was before [Jack] summoned me and gave me this job.
You'll have to read the book to figure out who Jack is, but I believe Zelzany's to be one of the freshest approaches to that legend, too.
My husband and I reread this book, a chapter each night, throughout this month. Max Gladstone wrote a fine appreciative post about it here.