Basically, anyone who's had anything beyond basic science classes in the US has worked in metric units. Once I got into high school, the only time that the English system was discussed was when my physics teacher ridiculed it by showing how complicated the English system units for various simple metric units were. Also, the names are humorous. The English system unit for mass (as opposed to weight) is a "slug" (which is the only thing I remember from that lecture).
RE #14: Have those books been translated or were they released in GB? If so, I'd guess that the translator/publisher made that change. Authors writing for the typical US market wouldn't use Celsius unless they did so for a particular effect -- and that would be seldom.
Re #13: By the late 70s, my state had even put up pairs of distance signs on the interstate to help people get used to the metric system in a practical application.
First sign: (Name of city) 161 km/100 miles
Second sign: (Name of city) 100 km/62 miles
It was at the same time that US automakers had to start making their analog (basically the only kind back then) speedometers show both miles and km.
It wasn't just the public that was against metrification. Ronald Reagan actively opposed it as president, even making the ridiculous claim the the dual speedometers were costing Detroit a huge some of money. [Once the change was made to the silkscreen (or whatever process was used to print the numbers on the speedometers), the only additional expense would be printing the boards in a second color. For Detroit, that's nothing.] By the time Reagan was out of office, the dual distance signs were down -- but apparently Detroit didn't find the speedometers to be too expensive...they remained.