It’s one of those books where the more you put into it, the more you get out of it. It’s full of insight about the design process, but I’ll summarise the main points.
Form and context
Christopher Alexander splits the design problem into two - form and context. The form is what we’re currently concerned with designing (take for instance a kettle). Everything else in the surrounding world which effects the form is the context (the kitchen surface, the user etc).
The aim of design is to make the form fit the context as well as possible.