A Pattern Language is applicable when putting lines on paper or nails in wood, i.e. actually designing and/or building. Though interesting, A Timeless Way of Building tends to be mostly useful for complaining about the way something has been built. Nobody talks about the third book in the trilogy The Oregon Experiment because the resulting buildings don't look like what people want to be the result of A Timeless Way of Building.

Anyway, in the actual practice of architecture, A Pattern Language is by far the best of the set. There are other books that make approximately the same case as Timeless Way of Building. [1] Most of it's fame is from standing in relation to A Pattern Language.

[1] e.g. How Buildings Learn, The Timeless Way of Seeing, Structure of the Ordinary, etc.