Alexander’s Notes on the Synthesis of Form also seems interesting; from the introduction:

The following argument is based on the assumption that physical clarity cannot be achieved in a form until there is first some programmatic clarity in the designer’s mind and actions; and that for this to be possible, in turn, the designer must first trace his design problem to its earliest functional origins and be able to find some sort of pattern in them. I shall try to outline a general way of stating design problems which draws attention to these functional origins, and makes their pattern reasonably easy to see.

It is based on the idea that every design problem begins with an effort to achieve fitness between two entities: the form in question and its context. The form is the solution to the problem; the context defines the problem. In other words, when we speak of design, the real object of discussion is not the form alone, but the ensemble comprising the form and its context. Good fit is a desired property of this ensemble which relates to some particular division of the ensemble into form and context.

It’s important to have a clear (but by no means fixed?) vision. In the next paragraph, he gives many examples of such ensembles: “The biological ensemble made up of a natural organism and its physical environment […]. [A] game of chess, where at a certain stage of the game some moves are more appropriate than others because they fit the context of the previous moves more aptly. The ensemble may be a musical composition – musical phrases have to fit their contexts too: think of the perfect rightness when Mozart puts just this phrase at a certain point in a sonata.” And so on.