Add "Scheme 9 from Empty Space" (#6603)

* Add "Scheme 9 from Empty Space"

* Edit "Scheme 9 from Empty Space"

* Edit URL
* Add year
* Add download links
Co-authored-by: NDavid Ordás <3125580+davorpa@users.noreply.github.com>

* remove source code links (moved to unglue.it)
Co-authored-by: NDavid Ordás <3125580+davorpa@users.noreply.github.com>
Co-authored-by: NEric Hellman <eric@hellman.net>

books/free-programming-books-langs.md

@@ -2222,6 +2222,7 @@ That section got so big, we decided to split it into its own file, the [BY SUBJE
 * [A Pamphlet Against R. Computational Intelligence in Guile Scheme](https://panicz.github.io/pamphlet/)
 * [An Introduction to Scheme and its Implementation](http://www.cs.rpi.edu/academics/courses/fall00/ai/scheme/reference/schintro-v14/schintro_toc.html)
 * [Concrete Abstractions: An Introduction to Computer Science Using Scheme](https://gustavus.edu/+max/concrete-abstractions.html) - M. Hailperin, B. Kaiser, K. Knight
+* [Scheme 9 from Empty Space - First edition (2007)](https://unglue.it/work/506723/) - Nils M. Holm (PDF)
 * [Scheme Tutorial](http://www.cs.hut.fi/Studies/T-93.210/schemetutorial/)
 * [Simply Scheme: Introducing Computer Science](http://www.cs.berkeley.edu/~bh/ss-toc2.html) - B. Harvey, M. Wright
 * [Teach Yourself Scheme in Fixnum Days](https://ds26gte.github.io/tyscheme/index-Z-H-1.html)