Try:
using (SqlConnection con = new SqlConnection(strConnect))
{
con.Open();
using (SqlCommand cmd = new SqlCommand("UPDATE myTable SET Quantity = Quantity + 1 WHERE BookID = @BI", con))
{
cmd.Parameters.AddWithValue("@BI", IdValueForTheRowToSayWhichBookItIs);
cmd.ExecuteNonQuery();
}
}
[EDIT]Forgot the WHERE condition...[/EDIT]