Added info to two books in Forth section (#12991)

Author: margaret-hu

books/free-programming-books-langs.md

@@ -792,8 +792,8 @@ Books on general-purpose programming that don't focus on a specific language are
 * [Easy Forth](https://skilldrick.github.io/easyforth/) - Nick Morgan (HTML)
 * [Forth Guide](https://web.archive.org/web/20110412231747/http://theforthsource.com/guide.html) - Glen B. Haydon (HTML) *( :card_file_box: archived)*
 * [Starting Forth](http://home.iae.nl/users/mhx/sf.html)
-* [Thinking Forth](http://thinking-forth.sourceforge.net)
-* [Thoughtful Programming and Forth](http://www.ultratechnology.com/forth.htm)
+* [Thinking Forth](http://thinking-forth.sourceforge.net) - Leo Brodie (HTML) (CC BY-NC-SA)
+* [Thoughtful Programming and Forth](http://www.ultratechnology.com/forth.htm) - Jeff Fox (HTML)
 
 
 ### Fortran