Add "Unix System Programming with Standard ML" (#6668)

books/free-programming-books-langs.md

@@ -2340,6 +2340,7 @@ That section got so big, we decided to split it into its own file, the [BY SUBJE
 * [Programming in Standard ML, DRAFT](http://www.cs.cmu.edu/~rwh/isml/book.pdf) - Robert Harper (PDF)
 * [SML# Document](http://www.pllab.riec.tohoku.ac.jp/smlsharp/docs/3.0/en/manual.xhtml) - Atsushi Ohori, Katsuhiro Ueno
 * [The Definition of Standard ML (Revised)](http://sml-family.org/sml97-defn.pdf) - SMLFamily GitHub project (PDF)
+* [Unix System Programming with Standard ML](http://mlton.org/References.attachments/Shipman02.pdf) - Anthony L. Shipman (PDF)
 
 
 ### Subversion