Nu har det tillkommit en source med data för demo

Books.ConsoleApp/Output.cs

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+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading.Tasks;
+
+namespace Books.ConsoleApp
+{
+    public class Output
+    {
+        private static List<BooksByAuthor> BooksByAuthorCatalog;
+
+        public static void BooksByAuthor(IEnumerable<Book> books)
+        {
+            BooksByAuthorCatalog = new List<BooksByAuthor>();
+
+            foreach(var book in books)
+            {
+                if (AuthorIsAlreadyCataloged(book.author))
+                {
+                    // there are some(1 or more) books by this author already found and catalogued
+                    var authorAndBooks = LocateAuthorAlreadyCataloged(book.author);
+                    authorAndBooks.Books.Add(book);
+                }
+                else
+                {
+                    CatalogueNewAuthor(book);
+                }
+            }
+
+            // now we have an list that has all the authors catalogued
+            OutputBooksByAuthor();
+        }
+
+        private static bool AuthorIsAlreadyCataloged(string author)
+        {
+            return BooksByAuthorCatalog.Any(ba => ba.Author == author);
+        }
+
+        private static BooksByAuthor LocateAuthorAlreadyCataloged(string author)
+        {
+            return BooksByAuthorCatalog.First(ba => ba.Author == author);
+        }
+
+        private static void CatalogueNewAuthor(Book b)
+        {
+            // there are NONE books by this author already found and cataloged
+
+            var newBooksList = new List<Book> { b };
+            var authorAndBooks = new BooksByAuthor(b.author, newBooksList);
+
+            BooksByAuthorCatalog.Add(authorAndBooks);
+        }
+
+
+        private static void OutputBooksByAuthor()
+        {
+            foreach(var ba in BooksByAuthorCatalog)
+            {
+                System.Console.Write("Author: {0,-28} Books: ", ba.Author);
+                foreach (var book in ba.Books)
+                {
+                    System.Console.Write(book.title + ", ");
+                }
+                System.Console.Write(Environment.NewLine);
+            }
+        }
+    }
+
+    public class BooksByAuthor
+    {
+        public readonly string Author;
+        public readonly List<Book> Books;
+
+        public BooksByAuthor(string author, List<Book> books)
+        {
+            Author = author;
+            Books = books;
+        }
+    }
+}