Sixty pages cover forty five things (there are some double and triple page spreads, and a removable four page poster). Many are actual machines — the International Space Station ("shared space house"), a dishwasher ("box that cleans food holders"), a camera ("picture taker") — and others are natural systems or their representations — the solar system ("worlds around the Sun"), weather maps ("cloud maps"), the electromagnetic spectrum ("colors of light") — and a few are other kinds of human constructs — the United States constitution, units of measurement ("how to count things").
I learned a few things from Thing Explainer, most notably about items whose operation I'd never thought about, such as dishwashers. There's also a kind of reverse-engineering challenge, working backwards from the explanations to the common technical terms. This is the opposite of having to look up complicated terms — and Munroe's own motivation was to liberate himself from the fear of using the wrong terminology.
My primary motivation for reading Thing Explainer was the thought that it might be pedagogically interesting, since I have an increasingly curious three year old. It's not directly useful when it comes to small children, since it avoids many quite concrete words they have no problem with (including such terms as "bridge" and "pipe", rendered here variously depending on context) and in any event isn't really pitched at pre-literate children, since there's a lot of rather small text. It has been indirectly useful, however, in making me think about my approach to explanation. It is too easy to respond to "what is that?" questions with the introduction of a new noun that simply labels an object or phenomenon instead of with an explanation that addresses its function, purpose or context.
Ultimately Thing Explainer is a kind of Oulipian exercise, combining complete freedom — Munroe inserts funny stick figures in fairly random places, and there are some puns — within rigid formal constraints. It manages to be informative while at the same time exploring the nature of language and terminology.
The large format of Thing Explainer makes the oversize diagrams with small print accessible. And this is a work where print is essential: an ebook version would be painful if not unusable, at least on most reading devices, and there doesn't appear to be an online version. This is a contrast to Munroe's other books, XKCD: 0 and What If?, both of which seem relatively pointless since their content works just as well if not better online.