I wouldn't read an educational material about how a submarine works, a helicopter works, and a microwave oven works all on the same day. I don't imagine many people would - especially if it were more technically laden in its vocabulary.

Likewise - I wouldn't read all of 'Thing Explainer' in a single sitting either. A page every now and then and you don't get bogged down with reading simple language for hours on end.

Reading over simplified things gets as tiring as reading overly complex material - because both can be quite mentally taxing. The increased difficulty of reading "eight and one" instead of "nine" gets more tiring than one might think...

Agree that it is not really a cover-to-cover book, but it works very well as a coffee table book to browse through when you are bored. We have a copy at our office, and our guests love it.