In The Image of the City, on how people understand and wayfind in cities, Kevin Lynch introduces the concept of imageability (how easy it is for a dialogue between the person and the environment to build into a good mental image) [notes], and five basic elements of these images: paths, edges, districts, nodes and landmarks [notes]. The book is brilliant; Lynch introduces a whole vocabulary for those emergent properties of human wiring and social habitation, then applies and explains. It's going to be enormously useful in thinking about how people learn to find their way around websites (and semantic spaces of all kinds), how we relate to space in general, and, more, how that space is collaboratively created and moulded. This is a modest book, self assured but not declarative or over-confident, quiet. A joy to read. (I also have notes on the book design.)