This is great. Are there such books written in Scheme? I am new to programming and still very ignorant, but in my experience so far Scheme seems to be unbelievably simple and easy. How come people prefer these clunky, unwieldy, unruly and, frankly speaking, scary languages like Java?

Not scheme, but ocaml : https://ocaml.github.io/ocamlunix/

You can try The Little Schemer

Is it about Systems Programming?

It's about Scheme and functional programming in general

GitHub Wiki? Why not WikiBooks, which is designed for this purpose?

It's more a FAQ than a book.

People now use GitHub not only to host codes, but also host legal documents, write blogs, write wiki books, as a discussion forum (issues). I also use GitHub to host SVG images.

It seems a bit hackish though, as though GitHub is not designed for these unorthodox usages.

GitLab CEO here, I think git tools will get better for non-programming use. GitHub is adding a lot of file formats (3D files, etc.) and people are creating book and law making tools with GitLab (see the section 'Build with GitLab' on https://about.gitlab.com/applications/ for examples). I do believe the interface needs to be simpler, but sites like PenFlip are already experimenting with an easy to understand interface.