In the second note it says" please burn the cipher message,which can be of no use of you" Why it was of no use?

The cipher message is believed to be of no use because it cannot be deciphered.

“But what is the use of a cipher message without the cipher?”

“In this instance, none at all.”

“Why do you say ‘in this instance’?”

“Because there are many ciphers which I would read as easily as I do the apocrypha of the agony column: such crude devices amuse the intelligence without fatiguing it. But this is different. It is clearly a reference to the words in a page of some book. Until I am told which page and which book I am powerless.”