The "certain unofficial manga" is a commercial product, while this is a self-published non-commercial doujinshi. A doujinshi based on a fanwork... in other words, as unofficial as it gets. It has no connection whatsoever to any official works, publishing companies or manga websites.

This is a collection of design references for characters who appear in "a certain unofficial manga". You can enjoy it as an artbook even without having read said manga, but it includes spoilers up to the very end of the story and things you may not have the context for. Be warned if you are planning to read it in the future.