There is no time difference between the two expressions. I agree with your book: "will you" is slightly more in the nature of a command, and "would you" of a request. But this difference is small and subtle, and the actual value of the expression depends on the social relationship between the speaker and the addressee.
For example, if I say to my wife "Will you post this letter?" I am asking her, because I have no right to order her. But if I happen to have a servant and say the same thing to him, it is much more in the nature of a command.
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