diff --git a/gradebook/src/GradeBook/Book.cs b/gradebook/src/GradeBook/Book.cs
index c4c438d..f5d95bc 100644
--- a/gradebook/src/GradeBook/Book.cs
+++ b/gradebook/src/GradeBook/Book.cs
@@ -19,7 +19,15 @@ namespace GradeBook
         }
     }
 
-    public abstract class Book : NamedObject
+    public interface IBook
+    {
+        void AddGrade(double grade);
+        Statistics GetStatistics();
+        string Name { get; }
+        event GradeAddedDelegate GradeAdded;
+    }
+
+    public abstract class Book : NamedObject, IBook
     {
         // Abstract class used to act as entrypoint for other class, so output 
         // can change based on given input. At this level, we cannot provide implementation
@@ -30,8 +38,14 @@ namespace GradeBook
 
         }
         public abstract void AddGrade(double grade);
+
+        public virtual event GradeAddedDelegate GradeAdded;
+        public virtual Statistics GetStatistics()
+        {
+            throw new NotImplementedException();
+        }
     }
-    public class InMemoryBook : Book
+    public class InMemoryBook : Book, IBook
     {
         private List<double> grades;
 
@@ -81,9 +95,9 @@ namespace GradeBook
             }
         }
 
-        public event GradeAddedDelegate GradeAdded;
+        public override event GradeAddedDelegate GradeAdded;
 
-        public Statistics GetStatistics()
+        public override Statistics GetStatistics()
         {
             var result = new Statistics();
             result.Average = 0.0;