string query = "DELETE FROM Books WHERE Title=@title";
"@title" is the parameter, and you give the @title param the relevant value here
command.Parameters.AddWithValue("@title", deletetitletxtbox.Text);
Next ExecuteScalar is for a command that returns a single column in a single row. The delete statement doesn't return anything so use ExecuteNonQuery instead, that is for when you don't expect any results back.