docs: add link for 'magic of css' e-book (#4052)

free-programming-books.md

@@ -1467,6 +1467,7 @@ Kerridge (PDF) (email address *requested*, not required)
 * [Learn CSS Layout](http://learnlayout.com)
 * [Learn CSS Layout the pedantic way](http://book.mixu.net/css/)
 * [Learn to Code HTML & CSS](https://learn.shayhowe.com) - Shay Howe
+* [Magic of CSS](https://adamschwartz.co/magic-of-css/) - Adam Schwartz (HTML) (:construction: *in process*)
 * [MaintainableCSS](http://maintainablecss.com)
 * [Pocket Guide to Writing SVG](https://svgpocketguide.com) - Joni Trythall
 * [Pro HTML5 Programming](https://web.archive.org/web/20181215200026/http://apress.jensimmons.com/v5/pro-html5-programming/ch0.html) - Jen Simmons, Chris O'Connor, Dylan Wooters, Peter Lubbers