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/Entities/Book.cs

@@ -0,0 +1,65 @@
+namespace LibraryApp.Domain.Entities;
+
+public class Book
+{
+    public BookId Id { get; private set; }
+    public BookTitle Title { get; private set; }
+    public BookAuthor Author { get; private set; }
+
+    public Book(BookTitle title, BookAuthor author) 
+        : this(new BookId(string.Empty), title, author)
+    { }
+
+    public Book(BookId id, BookTitle title, BookAuthor author)
+    {
+        Id = id;
+        Title = title;
+        Author = author;
+    }
+
+    public Book UpdateTitle(string newTitle)
+    {
+        Title = new (newTitle);
+        return this;
+    }
+    public Book UpdateAuthor(string newAuthor)
+    {
+        Author = new (newAuthor);
+        return this;
+    }
+}
+
+public record BookId : DbId
+{
+    public BookId(string id) : base(id) {}
+}
+
+public record BookTitle
+{
+    public string Title { get; init; }
+
+    public BookTitle(string title)
+    {
+        if(string.IsNullOrWhiteSpace(title))
+        {
+            throw new ArgumentException("Title cannot be empty.");
+        }
+        Title = title;
+    }
+}
+
+public record BookAuthor
+{
+    public string Author { get; init; }
+
+    public BookAuthor(string author)
+    {
+        if(string.IsNullOrWhiteSpace(author))
+        {
+            throw new ArgumentException("Author cannot be empty.");
+        }
+        Author = author;
+    }
+}
+
+