The mechanic's shop envisions the new GIS user as an apprentice mechanic entering a tool-filled garage for the first time. The new apprentice mechanic (GIS technician) will not, of course, attempt to learn all the tools in the shop at once. Instead, he or she will be guided in the use of a select few of the most useful and fundamental. In GIS we learn first how to stack and symbolise layers, how to run simple buffers or do basic spatial analysis. We learn how to make simple and effective maps.
The overlay metaphor is part of the founding myth of GIS, in which an architect seeking an ideal route through several diverse sites, attribute fields and land uses, stacks translucent map overlays on top of each other and maps (draws) a route through areas in which several basic criteria are met. This metaphor/myth is borrowed from Nadine Schuurman's GIS book. During lecture I had recommended this book as one of the best, if not the best, on critical GIS. Having been published in 2004, however, it doesn't cover the geoweb material as well and, in the end, this is why it did not become required reading.
Schuurman's book is well worth checking out, and Bedford Library has two copies.