Biggest problem is, CSV is not a standardized format like JSON. For very simple cases it could be used as a database like format. But it depends on the parser and that’s not ideal.

JSON is easier to parse, smaller and lighter on resources. And that is important in the web. And if you take into account all the features XML has, plus the entities it gets big, slow and complicated. Most data does not need to be self descriptive document when transferring through web. Fundementally these languages are two different kind of languages: XML is a general markup language to write documents, while JSON is a generalized data structure with support for various data types supported by programming languages.

books is a specific type. library is just a different word for collection. So calling them books would be wrong. Because library does not imply a book. In example you can have a library of videos or a library of images.

Here is a video about this subject: https://inv.nadeko.net/watch?v=hymd3Xc7cCU This channel (SavvyNik) often directly reads and shows parts of the original mailing list source, and if available the relevant part of the interview in video form.

Maybe its not a bad idea to experiment with the tools that lot of people do, so it might help understanding what code it produces. That might be not his original goal, but its a nice side effect I guess.