Enough already. I'm obviously not incentivized/shamed into writing unit tests for things in UNTESTED dir.s. It's just making a mess of the file tree

DataLayer/EfClasses/LibraryBook.cs

@@ -0,0 +1,33 @@
+using System;
+using Dinah.Core;
+
+namespace DataLayer
+{
+    public class LibraryBook
+    {
+        internal int BookId { get; private set; }
+        public Book Book { get; private set; }
+
+        public DateTime DateAdded { get; private set; }
+
+        // immutable-ish. should be immutable. mutability is necessary for v3 => v4 upgrades
+        public string Account { get; private set; }
+
+        private LibraryBook() { }
+        public LibraryBook(Book book, DateTime dateAdded, string account)
+        {
+            ArgumentValidator.EnsureNotNull(book, nameof(book));
+            ArgumentValidator.EnsureNotNull(account, nameof(account));
+
+            Book = book;
+            DateAdded = dateAdded;
+            Account = account;
+        }
+
+        // needed for v3 => v4 upgrade
+        public void UpdateAccount(string account)
+            => Account ??= account;
+
+        public override string ToString() => $"{DateAdded:d} {Book}";
+	}
+}