diff --git a/free-programming-books.md b/free-programming-books.md
index a490fc765d039113a024d7d74d2b404cd4d0e206..b3e2fec7be77f531e609e26fe82efe8e4268b66e 100644
--- a/free-programming-books.md
+++ b/free-programming-books.md
@@ -659,6 +659,7 @@ Kerridge (PDF) (Just fill the fields with any values)
 * [How to deal with Passwords](https://github.com/MHM5000/pass)
 * [Intrusion Detection Systems with Snort](http://ptgmedia.pearsoncmg.com/images/0131407333/downloads/0131407333.pdf) (PDF)
 * [OpenSSL Cookbook](https://www.feistyduck.com/library/openssl-cookbook/)
+* [OWASP Testing Guide 4.0](https://www.owasp.org/images/1/19/OTGv4.pdf) (PDF)
 * [OWASP Top 10 for .NET Developers](http://www.troyhunt.com/2011/12/free-ebook-owasp-top-10-for-net.html)
 * [Security Engineering](http://www.cl.cam.ac.uk/~rja14/book.html)