I think some of these aren't North Korean-authored, but international textbooks in unofficial translation. For example, Google matches code samples from "CTBooks1/ANSI_C++.pdf" to this textbook from 1999 (and the front-cover graphic is identical),

https://books.google.com/books/about/Object_oriented_Softwar...

I think the version they're translating is a UK print edition, because the currency unit in the code is "£".

But the series about Red Star OS [0] are very certainly DPRK-authored.

[0] https://en.wikipedia.org/wiki/Red_Star_OS

The music textbooks are worth noticing (they're very obviously North Korean -- e.g. "Lesson 1. Song of Kim Il-sung"). It's all in Western notation. References both Western and Korean instruments. There's bow markings on the score of "Song of Kim Il-sung", so I infer that violins are at least somewhat common in secondary schools (or why else are they there?)