1976 book discussing the commercial possibilities of colonising interplanetary space; O'Neill's thesis is that there's no real point in clawing our way out of this gravity well just to go and live in another one, when there's so much interesting stuff to do off-world. (At least one of the SF series I've been reading recently -- Reynolds, I think -- calls its space stations "oneills" in homage to this book.)
The downside is that this is a terribly depressing book to read in 2007, because we don't have any of the cool stuff that O'Neill thought we'd be good at building by the turn of the millennium...
ISBN 189652267X.