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My English grammar books says:

You need not come unless you want to. -Incorrect

You don’t need to come unless you want to. -Correct

It does not provide the reason for (it) or cause of it though. I wonder why.

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    Your English grammar book is simply wrong. As LawrenceC says, treating "need" as a modal is no longer common in American English (which after all is much more English than English English), but it is perfectly acceptable in educated written English. Where your grammar book is correct is that the usage is now very rare in colloquial American English, so rare that it may not be understood in some circles and viewed as affected in other circles. So, it is fine in formal writing, but to be avoided otherwise. – Jeff Morrow Dec 18 '17 at 14:46
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Need not is fine, but it does sound a little formal or British (I'm an AmE speaker). Need can be a semi-modal verb.

Similar structures with real modals are common:

You ought/must/should not come unless ...

"You don't need" is more idiomatic in everyday AmE speech.

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You need not come unless you want to.

You don't need to come unless you want to.

There's nothing wrong with these sentences. If you follow BrE, you can use either form: needn't/need not or don't need to/do not need to.

But Americans don't normally use the word need as a modal verb i.e. needn't/you need not.

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