These are over twenty years old, so I wonder why I never came across them before. Good aeroplane-trip reading.
A Layered Approach to Software Design (Ira Goldstein and Daniel Bobrow, CSL-80-5, Xerox PARC, December 1980) 1.9 MB PDF

Following the receipt of contributions from a collaborator, the need arises to select some of these proposals and combine them... A design environment should make it easy to examine overlapping designs and to select pieces for combination into a joint design. A layered design facilitates this since coordinated sets of change have been localized into layers.