I read a sci-fi book about 20+ years ago (book was probably older than that though) about earths future where there are all different guilds. One guild was low in numbers and the need for them was apparently ending (or so people thought). They were the watchers who kept a vigil for an invasion. The main character was one of these final watchers who, to the best of my memory, pushed a hot dog cart looking thing around that he later used to signal the invasion was coming from tear dropped shaped ships. (I think he may have made a false alarm at one point before the real invasion) Thats about all I can remember. Hope it rings a bell with some sci-fi types.
Just a guess: Nightwings. Robert Silverberg
http://www.amazon.com/Nightwings-Robert-Silverberg/dp/0743444744