I am reading Concrete Mathematics now and found a printing error,I have sent email to him.
How long the email will be got reply?I heard that he do not use email now,is it necessary to write a snail mail?
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Knuth's web page for Concrete Mathematics explains very specifically how to contact him regarding errors in the text, and lists those which are already known.
His web page also has a FAQ about rewards. Due to his concerns about personal checks, he does not send out real money anymore. Instead, rewards are now "deposited" to a fictitious bank whose "account balances" are listed on the web.
So you still get the notoriety, even without the cash.