This was a book I had to purchase for school. It was informative. At that time json was gaining popularity but there was still a lot of XML in use.
At the time of learning it, I thought it was really interesting you could define your own markup language and make it do whatever you want (like creating your own version of html). Turns out we ended up doing that later on but call them components and the tooling is much nicer than writing schemas and complex parsers..