Rybczynski examines the theories of many thinkers at the forefront of urbanism, especially Jane Jacobs and Lewis Mumford, but also Charles Mulford Robinson, Ebenezer Howard, and Le Corbusier. With the benefit of retrospect in many instances, the author looks at where the ideas of these thinkers have worked in contemporary cities, and where they haven't.
There are many issues facing cities today, and in the latter parts of the book Rybczynski looks at where cities might need to go in the future to remain places people can and will want to live. The answer, in the mind of the author, is density. Not necessarily hyperdensity, as with cities like New York City and Hong Kong (although there are lessons to be learned from those examples), but simply an increased density compared to what is seen in most cities today. This is particularly important in Ottawa, where a relatively low population density offers ample opportunity for the city to develop within the city without needing to move out of it--an obvious reason why City Council has worked hard to limit and altogether avoid expanding the urban boundary.
In the end, Rybczynski concludes that good planning makes good cities, citing the Israeli city of Modi'in as proof. But rather than designing every detail of the city, Rybczynski calls for macro-planning, allowing for organic growth over time under parameters set forth. It's not the hyper-planned city of Le Corbusier, nor the anarchistic city of Jacobs, but something in between.