diff --git a/book/chapter-06-gbapp-reference.md b/book/chapter-06-gbapp-reference.md
index 144eb913..315015c6 100644
--- a/book/chapter-06-gbapp-reference.md
+++ b/book/chapter-06-gbapp-reference.md
@@ -219,3 +219,6 @@ This in some way contributes to a better understanding of how it works.
 | npm update -g | Updates NPM                         |
 | node -v       | Checks node version                 |
 | ncu -a        | Update all packages in package.json |
+
+## Useful documentation 
+-https://www.typescriptlang.org/docs/handbook (TypeScript Documentation).