Added "Practical Foundations for Programming Languages, Preview"

free-programming-books.md

@@ -226,6 +226,7 @@
 * [Introduction to Theory of Computation](http://cg.scs.carleton.ca/~michiel/TheoryOfComputation/) (PDF) - Anil Maheshwari and Michiel Smid
 * [Models of Computation](http://cs.brown.edu/people/jes/book/) - John E. Savage
 * [Network Science](http://barabasilab.neu.edu/networksciencebook/index.html)
+* [Practical Foundations for Programming Languages, Preview](http://www.cs.cmu.edu/~rwh/plbook/book.pdf) - Robert Harper
 * [Programming Languages: Application and Interpretation (2nd Edition)](http://www.cs.brown.edu/~sk/Publications/Books/ProgLangs/)
 * [Structure and Interpretation of Computer Programs](http://mitpress.mit.edu/sicp/)
 * [Think Complexity](http://www.greenteapress.com/compmod/) - Allen B. Downey