diff --git a/HACKING b/HACKING
index 789b7db82e..0fd1615d81 100644
--- a/HACKING
+++ b/HACKING
@@ -160,7 +160,9 @@ understanding. A reference manual for Xlib can be found e.g.
 at ftp://ftp.x.org/pub/X11R7.0/doc/PDF/xlib.pdf , a tutorial explaining basic can be found
 e.g. at http://users.actcom.co.il/~choo/lupg/tutorials/xlib-programming/xlib-programming.html
 (note that you don't need to know that all - e.g. GC's are very rarely needed and the
-section on fonts is today outdated).
+section on fonts is today outdated). At http://www.sbin.org/doc/Xlib/index_contents.html can
+be found an Xlib programming manual (some of those are more or less obsolete these days, but
+there is e.g. an extensive section related to window management).
 
 
 Coding style: