Updated the domain model with entities, repos and domain services. Looks good. I think I addressed all the user stories however if I fell short we'll revise as needed.

LibraryApp.Domain/Error.cs

@@ -0,0 +1,14 @@
+namespace LibraryApp.Domain;
+
+public record Error
+{
+    public string Message { get; init; }
+
+    public Error(string message)
+    {
+        Message = message;
+    }
+
+    public static implicit operator Error(string message) => new Error(message);
+    public static implicit operator string(Error error) => error.Message;
+}