diff --git a/gradebook/src/GradeBook/Book.cs b/gradebook/src/GradeBook/Book.cs
index f5d95bc..58a0e03 100644
--- a/gradebook/src/GradeBook/Book.cs
+++ b/gradebook/src/GradeBook/Book.cs
@@ -1,4 +1,5 @@
 using System;
+using System.IO;
 using System.Collections.Generic;
 
 namespace GradeBook
@@ -39,8 +40,31 @@ namespace GradeBook
         }
         public abstract void AddGrade(double grade);
 
-        public virtual event GradeAddedDelegate GradeAdded;
-        public virtual Statistics GetStatistics()
+        public abstract event GradeAddedDelegate GradeAdded;
+        public abstract Statistics GetStatistics();
+    }
+
+    public class DiskBook : Book
+    {
+        public DiskBook(string name) : base(name)
+        {
+        }
+ 
+        public override event GradeAddedDelegate GradeAdded;
+
+        public override void AddGrade(double grade)
+        {
+            using(var writer = File.AppendText($"{Name}.txt"))
+            {
+                writer.WriteLine("Test");
+                if (GradeAdded != null)
+                {
+                    GradeAdded(this, new EventArgs());
+                }
+            }
+        }
+
+        public override Statistics GetStatistics()
         {
             throw new NotImplementedException();
         }