Published in 1959, it was the seminal work of Kevin Lynch, an MIT professor in the school of Architecture and Planning.  His book is about the way cities look, or rather the mental picture that we all form in our heads of cities. His book looks at three American cities: Boston, Jersey City (Part of the New York metropolitan area), and Los Angeles.
So, we are surveying the city much like an artist might survey a piece of art, it brings to mind the Goethe quote that architecture is like “frozen music”. It differs however, in that unlike the semi-quaver on the page, or paint on the canvas, cities lack such controlled parameters; they vary, being affected by time, weather, and the interactions of those who populate them.
Cities engage all our senses. We associate particular areas with our own experiences within said areas - one part is understood in relationship to what is adjacent, there is sequential development bringing things into their present form. The city's image in our mind is rich and multifaceted.
Lynch talks about the “legibility” of cities, meaning the extent to which a city can be conceptualized in a coherent way, how easily can it be understood, and in turn, how easily can it be navigated through and engaged with. A psychological point is made: when one talks of orientating themselves, they may be speaking in literal terms or metaphorical terms. One may orientate themselves in relation to an idea, an expectation, a hope, etc. A sense of being lost in literal terms relates to the metaphorical, a sense of lostness, of disorientation, of anxiety building. Ultimately cities are not cold tools, they are lived within, imbued with memories and meaning. Meaning which is not just held by individuals, but also held collectively.
Lynch points out the obvious, that though we may wish to strive for “legible” cities, cities which are aesthetically pleasing, America seems pretty adept at creating urban environments that fall woefully short. So much so, that that they rob Americans of even the ability to imagine what value cities might hold. Ultimately cities are never done, however, they are always in process, always capable of being reimagined. He goes through his methodology for exploring the urban environments of the three cities in question; this is the least compelling part of the book for me, so I’ll skip it.
Okay, so we have the image of the city. In effect there is not one image, but one image which exists within every individual. And as a kind of emergent property these individual images derive the collective image, a collective image which is largely incommunicable. Although this collective image is shaped by such intangibles as social meaning, function, history, and names of areas, Lynch wishes to focus in on the role of particular physical forms. And this brings us to Lynch’s big five which are: paths, edges, districts, nodes, and landmarks.

1.      Paths are the channels along which people move, such as streets, walkways, and transit lines. Crucial as observers typically organize environmental elements along these paths.

2.      Edges are perceived boundaries between two phases, like railroads or shorelines, acting as linear breaks in continuity but not necessarily paths themselves.

3.      Districts are medium-to-large sections of the city recognized by some common, identifying character, offering a two-dimensional sense of being "inside" a particular area.

4.      Nodes are strategic focal points like squares or junctions where a person can enter, serving as the core from which travelers orient themselves.

5.      Landmarks are external point references that stand out due to their form, location, or symbolic value.

These are the elements that help individuals navigate and understand their urban environment. The urban planner reinforces a coherent city image by appropriately deploying these five elements. In conclusion, the book advocates for an approach to urban design that prioritizes clarity, coherence, and meaningful engagement with the city's forms. They highlight the potential of thoughtful design to transform urban spaces into places of beauty, significance, and enjoyment for their inhabitants.