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/Result.cs

@@ -0,0 +1,23 @@
+namespace LibraryApp.Domain;
+
+public record Result
+{
+    public required bool IsSuccess { get; init; }
+    public Error? ErrorMessage { get; init; }
+
+    private Result() { }
+
+    public static Result Success() => new Result { IsSuccess = true };
+    public static Result Failure(string errorMessage) => new Result { IsSuccess = false, ErrorMessage = errorMessage };
+}
+public record Result<T>
+{
+    public required bool IsSuccess { get; init; }
+    public T? Value { get; init; }
+    public Error? ErrorMessage { get; init; }
+
+    private Result() { }
+
+    public static Result<T> Success(T value) => new Result<T> { IsSuccess = true, Value = value };
+    public static Result<T> Failure(string errorMessage) => new Result<T> { IsSuccess = false, ErrorMessage = errorMessage };
+}