vb linq Count Distinct?

In the code bellow the NumberOfBooks does not return a count of distinct ISBNs. (If I have say 100 and 40 of them are the same ISBN I get a count of 100.) How can I have it return a count that would be based on Unique ISBN?

Dim results = (From r In dt.AsEnumerable()
                       Group By KeyEventArgs = "OneGroup" Into justOneGroup = Group
                       Select New With {
                    .NumberOfBooks = justOneGroup.Distinct.Count(Function(Books) Books.Field(Of String)("isbn")),
                    .CostPrice = justOneGroup.Sum(Function(c) c.Field(Of Decimal)("CostPriceTotal")),
                    .Qty = justOneGroup.Sum(Function(Books) Books.Field(Of Int32)("QtyLeft")),
                    .MaxQty = justOneGroup.Max(Function(Books) Books.Field(Of Int32)("MaxQty")),
                    .MinDate = justOneGroup.Min(Function(Books) Books.Field(Of Date)("FinalShipDate"))
            }).ToList()

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