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/DbId.cs

@@ -0,0 +1,13 @@
+namespace LibraryApp.Domain;
+
+public record DbId
+{
+    public string Id { get; init; }
+    public bool BlankId => string.IsNullOrWhiteSpace(Id);
+    public DbId(string id)
+    {
+        if(id is null)
+            throw new ArgumentNullException(nameof(id));
+        Id = id;
+    }
+}