Merge pull request #858 from diekmann/patch-2

Concrete Semantics - A Proof Assistant Approach

free-programming-books.md

@@ -901,6 +901,7 @@
 
 
 ###Isabelle/HOL
+* [Concrete Semantics - A Proof Assistant Approach by Tobias Nipkow and Gerwin Klein](http://www21.in.tum.de/~nipkow/Concrete-Semantics/) (PDF)
 * [Isabelle/HOL - A Proof Assistant for Higher-Order Logic by Tobias Nipkow and Lawrence C. Paulson and Markus Wenzel](http://isabelle.in.tum.de/doc/tutorial.pdf) (PDF)