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/Configurations/LibraryBookConfig.cs

@@ -0,0 +1,18 @@
+using Microsoft.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore.Metadata.Builders;
+
+namespace DataLayer.Configurations
+{
+    internal class LibraryBookConfig : IEntityTypeConfiguration<LibraryBook>
+    {
+        public void Configure(EntityTypeBuilder<LibraryBook> entity)
+        {
+            entity.HasKey(b => b.BookId);
+
+            entity
+                .HasOne(le => le.Book)
+                .WithOne()
+                .HasForeignKey<LibraryBook>(le => le.BookId);
+        }
+    }
+}