diff --git a/books/free-programming-books-subjects.md b/books/free-programming-books-subjects.md
index 2f09dde2a..6f8b6336d 100644
--- a/books/free-programming-books-subjects.md
+++ b/books/free-programming-books-subjects.md
@@ -60,6 +60,7 @@ Books that cover a specific programming language can be found in the [BY PROGRAM
 * [Atariarchives.org](https://www.atariarchives.org) - Atariarchives.org makes books, information, and software for Atari and other classic computers available on the Web.
 * [Bento](https://www.bento.io)
 * [Bitsavers.org](http://bitsavers.trailing-edge.com)
+* [Build your own X](https://github.com/codecrafters-io/build-your-own-x)
 * [Code Catalog: A Collection of Code Examples from Prominent Open Source Projects](https://codecatalog.org)
 * [conceptf1.blogspot.com](https://conceptf1.blogspot.com/2013/11/list-of-freely-available-programming.html)
 * [dBooks - Free download open books](https://www.dbooks.org)