a vs. the 3

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Dec 2, 2009 00:34 article
(a) If a reader is too immature to understand well what is written in #a# book, the only thing he can do is to wait for himself to grow up.
(b) If a reader is too immature to understand well what is written in #the# book, the only thing he can do is to wait for himself to grow up.

Which article is appropriate in this context?
Should I regard as important the point that the "book" is first mentioned and select "a", or think "a reader" already indicates the existence of the book and select "the"?



Thank you in advance!!!