Add TypeScript in 50 Lessons in books (#9136)

books/free-programming-books-langs.md

@@ -2475,6 +2475,7 @@ That section got so big, we decided to split it into its own file, the [BY SUBJE
 * [TypeScript Deep Dive](https://basarat.gitbooks.io/typescript/)
 * [TypeScript for C# Programmers](http://www.infoq.com/minibooks/typescript-c-sharp-programmers)
 * [TypeScript Handbook](https://www.typescriptlang.org/assets/typescript-handbook.pdf) - Microsoft (PDF)
+* [TypeScript in 50 Lessons](https://www.smashingmagazine.com/provide/eBooks/typescript-in-50-lessons.pdf) - Stefan Baumgartner (PDF)
 * [TypeScript Notes for Professionals](https://goalkicker.com/TypeScriptBook2/) - Compiled from StackOverflow documentation ([PDF](https://goalkicker.com/TypeScriptBook2/TypeScriptNotesForProfessionals.pdf))
 * [TypeScript Succinctly, Syncfusion](https://www.syncfusion.com/resources/techportal/ebooks/typescript) (PDF, Kindle) (email address *requested*, not required)