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It probably depends on the book and the sort of question. Why don't you write down the question here in meta and let people decide? At present there is not enough information to give a proper answer.
I think these kinds of questions are fine, as long as it's of the appropriate level (which I'm sure your question will be) and you explain your thought process so far - I think my question here is a good example. Though checking in meta first never hurts, I suppose.
One strategy is to ask for an explanation on math.SE first and, if one is not forthcoming, ask on MO later.
@Qiaochu: Whenever I've done that, my question on math.SE gets no votes and no answers. I agree with José that it really depends on the book.
Another place to ask if the subject seems like it's the right flavor is the nForum.
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