"Design for the Other 90%" was published for the exhibition of the same name at the Cooper-Hewitt National Design Museum at the Smithsonian. More like a review of an exhibition than a definitive text, the book is a nice brief review of some very interesting designs that are useful all over our planet. The book does not seem to be intended to teach you how to build, but rather to inspire you to think about building differently.
The book contains a collection of inventions - most of them quite simple. Examples are a "pot-in-pot" cooler which uses evaporative cooling to keep food from spoiling, a portable water filter for drinking water, a communal solar-powered kitchen, a gravity-powered drip irrigation system, and small-scale photovoltaic lighting. There are many more interesting ideas within these pages.
You won't learn the details of construction from this book, but you will be stimulated to think more broadly about designs that are useful for the majority of humans on our planet.