Rework book server and client.

examples/book_server/book.cc

@@ -0,0 +1,11 @@
+#include "book.h"
+
+#include <iostream>
+
+const Book kNullBook{};
+
+std::ostream& operator<<(std::ostream& os, const Book& book) {
+  os << "{ " << book.id << ", " << book.title << ", " << book.price << ", "
+     << book.photo << " }";
+  return os;
+}