The basic idea there is that economics and ecology have all kinds of things in common, such as differentiation emerging from generality, differentiations becoming generalities from which further differentiations emerge, developments depending on co-developments, bifurcations, positive feedback loops, negative-feedback controls, and emergency adaptations.
She doesn't explicitly say so, but the central message seems to be that any viable structure is constantly changing and reinventing itself, and that any static, unchanging living or economic structure is doomed. Sometimes I think she's stretching metaphors to the breaking point, but on the whole, I like the book.
The format of the book is what she calls a "dialogue," rather than regular non-fiction, but not in the sense of the classical didactic dialogues that went simply
No, it's more like an extremely dialogue-heavy novel where the only thing that happens is that the characters talk to each other about things the author finds interesting.GUY No. 1: I have this idea I'd like to share.
GUY No. 2: Tell me all about it!
More generally, one thing that's only tangentially mentioned in this book, but that I'd say is very central to her ideas in general, is the idea of adding new activities to existing activities. (In fact, that idea is so central to her work that I wonder why zompist didn't mention it at all in his otherwise great introduction to her ideas: http://zompist.com/jacobs.html)
In one of her earlier books (I think it was The Economy of Cities, but I might misremember that) she listed a lot of examples of adding new activities to existing activities; one that I still remember without having to look it up is the history of a business that had started out mining sand in quarries and selling it to industrial businesses for use in sandblasting.
After the while, the people in charge of that business decided that if they were already dealing in sand, they might as well start making and selling sandpaper. After all, they already had the sand, they would just have to glue it to hardened paper, and their history of selling sand for sandblasting meant that they already had a lot of business contacts with exactly the kind of industrial enterprises that might be interested in buying a lot of sandpaper.
Then, a while after they had started selling sandpaper, they got the next idea: if they already had paper with sand glued to it, they might as well leave out the sand, take just the paper with glue on one side, and cut it into stripes: voilà, adhesive tape.
And so, in two steps that make perfect sense in hindsight, but would probably have been impossible to predict in advance, a business that had started out selling sand from quarries had ended up selling adhesive tape.
Now, according to Jacobs, stuff like that is the essential engine of all viable economic development: if you want to get anywhere economically, you must do it that way. If true, that also offers an elegant additional explanation for why the 20th century attempts at centrally planning economies failed: you can't add new activities to existing activities in that way if the Five Year Plan tells you exactly, in meticulous detail, what you're supposed to do for the next five years.
Unrelated to economic matters, and perhaps of interest to people here, we can see a similar thing when the meanings of words shift semantically over time (this is my idea now, not hers, but based on her ideas): A word that means one thing takes on a different, but somehow related, second meaning; then, a while later, in another step, it takes on a third meaning that is still somehow related to the second meaning, but might have no recognizable relation to the first meaning.