public void Input(string Command)
 {
     Console.Clear();
     if (Command.Length != 0)
     {
         if (Command == _system.GetUser(Command).Username)
         {
             PrintListOfBooks(Command);
         }
         else
         {
             _cli.PrintUserDoesNotExist(Command);
         }
     }
     else
     {
         _cli.PrintMissingInput();
     }
 }