Habitué [Fr. a-BEE-twey]-- noun
1. Devotee 2. a frequent visitor to a place 3. denizen
Hm...I don't know for sure, but if I guessed, it would be that punctuation book that came out fairly recently (that I haven't read). Eats, Shoots and Leaves?
I googled it (because the internet knows everything) but it failed me. However, what I read about the concept makes me feel pretty guilty for using SO MANY "brackets" in my writing. I will try to do better.
It sounds similar to this line from _Eats, Shoots & Leaves_: "Writers who place whole substantive passages in brackets can't possibly appreciate the existential suffering they inflict" (162). It is a delightful, hilarious sentiment.
I did it in my comment and didn't even notice (I'm a failure!).