diff --git a/hash/examples/books.cpp b/hash/examples/books.cpp
index 00d41fc..e91ebba 100644
--- a/hash/examples/books.cpp
+++ b/hash/examples/books.cpp
@@ -3,37 +3,16 @@
 //  subject to the Boost Software License, Version 1.0. (See accompanying
 //  file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
 
+#include "./books.hpp"
 #include <boost/functional/hash.hpp>
-// If we had std::unordered_set.
-//#include <unordered_set>
 #include <cassert>
 
+// If std::unordered_set was available:
+//#include <unordered_set>
+
 // This example illustrates how to use boost::hash with a custom hash function.
 // For full details, see the tutorial.
 
-namespace library
-{
-    struct book
-    {
-        int id;
-        std::string author;
-        std::string title;
-
-        book(int i, std::string const& a, std::string const& t)
-            : id(i), author(a), title(t) {}
-    };
-
-    bool operator==(book const& a, book const& b)
-    {
-        return a.id == b.id;
-    }
-
-    std::size_t hash_value(book const& b)
-    {
-        return boost::hash_value(b.id);
-    }
-}
-
 int main()
 {
     library::book knife(3458, "Zane Grey", "The Hash Knife Outfit");
@@ -42,7 +21,7 @@ int main()
     boost::hash<library::book> book_hasher;
     std::size_t knife_hash_value = book_hasher(knife);
 
-    // If we had std::unordered_set:
+    // If std::unordered_set was available:
     //
     //std::unordered_set<library::book, boost::hash<library::book> > books;
     //books.insert(knife);
@@ -53,3 +32,17 @@ int main()
     //assert(books.find(knife) != books.end());
     //assert(books.find(dandelion) == books.end());
 }
+
+namespace library
+{
+    bool operator==(book const& a, book const& b)
+    {
+        return a.id == b.id;
+    }
+
+    std::size_t hash_value(book const& b)
+    {
+        boost::hash<int> hasher;
+        return hasher(b.id);
+    }
+}
diff --git a/hash/examples/books.hpp b/hash/examples/books.hpp
new file mode 100644
index 0000000..411b066
--- /dev/null
+++ b/hash/examples/books.hpp
@@ -0,0 +1,26 @@
+
+//  Copyright Daniel James 2005. Use, modification, and distribution are
+//  subject to the Boost Software License, Version 1.0. (See accompanying
+//  file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+// This example illustrates how to use boost::hash with a custom hash function.
+// The implementation is contained in books.cpp
+
+#include <cstddef>
+#include <string>
+
+namespace library
+{
+    struct book
+    {
+        int id;
+        std::string author;
+        std::string title;
+
+        book(int i, std::string const& a, std::string const& t)
+            : id(i), author(a), title(t) {}
+    };
+
+    bool operator==(book const&, book const&);
+    std::size_t hash_value(book const&);
+}