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/BookInstance.cs

@@ -0,0 +1,99 @@
+namespace LibraryApp.Domain.Entities;
+
+public enum BookInstanceStatus
+{
+    OnShelf,
+    Lent,
+    PutAside,
+    Lost
+}
+
+public class BookInstance
+{
+    public BookInstanceId Id { get; private set; }
+    public BookId BookId { get; private set; }
+    public ShelfId ShelfId { get; private set; }
+    public BookInstanceBarcode Barcode { get; private set; }
+    public BookInstanceStatus Status { get; private set; }
+
+    public BookInstance(BookId bookId, ShelfId shelfId, BookInstanceBarcode barcode)
+        : this(new BookInstanceId(string.Empty), bookId, shelfId, barcode, BookInstanceStatus.OnShelf)
+    { }
+
+    public BookInstance(BookInstanceId id, BookId bookId, ShelfId shelfId, BookInstanceBarcode barcode, BookInstanceStatus status)
+    {
+        Id = id;
+        BookId = bookId;
+        ShelfId = shelfId;
+        Barcode = barcode;
+        Status = status;
+    }
+
+    public BookInstance MarkAsLent()
+    {
+        if (Status != BookInstanceStatus.OnShelf)
+            throw new InvalidOperationException($"Cannot lend a book that is not on the shelf. Current status: {Status}.");
+        Status = BookInstanceStatus.Lent;
+        return this;
+    }
+
+    public BookInstance ReturnToShelf()
+    {
+        if (Status != BookInstanceStatus.Lent)
+            throw new InvalidOperationException($"Cannot return a book that is not lent out. Current status: {Status}.");
+        Status = BookInstanceStatus.OnShelf;
+        return this;
+    }
+
+    public BookInstance MarkAsPutAside()
+    {
+        if (Status != BookInstanceStatus.OnShelf)
+            throw new InvalidOperationException($"Cannot put aside a book that is not on the shelf. Current status: {Status}.");
+        Status = BookInstanceStatus.PutAside;
+        return this;
+    }
+
+    public BookInstance PlaceOnShelf(ShelfId shelfId)
+    {
+        if (Status != BookInstanceStatus.PutAside)
+            throw new InvalidOperationException($"Cannot place on shelf a book that is not put aside. Current status: {Status}.");
+        ShelfId = shelfId;
+        Status = BookInstanceStatus.OnShelf;
+        return this;
+    }
+
+    public BookInstance MarkAsLost()
+    {
+        if (Status != BookInstanceStatus.OnShelf)
+            throw new InvalidOperationException($"Cannot mark as lost a book that is not on the shelf. Current status: {Status}.");
+        Status = BookInstanceStatus.Lost;
+        return this;
+    }
+
+    public BookInstance UnmarkAsLost()
+    {
+        if (Status != BookInstanceStatus.Lost)
+            throw new InvalidOperationException($"Cannot unmark as lost a book that is not marked as lost. Current status: {Status}.");
+        Status = BookInstanceStatus.OnShelf;
+        return this;
+    }
+}
+
+public record BookInstanceId : DbId
+{
+    public BookInstanceId(string id) : base(id) {}
+}
+
+public record BookInstanceBarcode
+{
+    public string Barcode { get; init; }
+
+    public BookInstanceBarcode(string barcode)
+    {
+        if(string.IsNullOrWhiteSpace(barcode))
+        {
+            throw new ArgumentException("Barcode cannot be empty.");
+        }
+        Barcode = barcode;
+    }
+}