I just watched it too. The Ghost Dog comparisons are not without merit. I would like to add Le Cercle Rouge to the list. It is also by Melville and also stars Delon, so certainly worth picking up.

As for the film itself, Delon isn't the only actor that is emotionless, everyone is. It's a great stylized acting that is very familiar for French films of the period. Almost Bressonian in it's lack of emotion, although Bresson had an amazing way of coaching actors even without changing expressions. The film is quiet, moving, and how about that ending? I'm still trying to figure it out.