I went and downloaded them all when they first showed up (also free in Canada, BTW).
They're very short, but not bad. Kind of like a hybrid of a decently-written Wikipedia article and the
BBC Schools revision stuff at the "Scottish Highers" level. A nice, light introduction with a little depth and detail to most of the subjects I tried.
Nowhere near worth the $1.99 "regular" asking price (unless they consolidated all the ones on related subjects into one single "TextVook"), but definitely good enough to download and read as freebies.