How do I redownload my audibooks from the iTunes store? My hard drive crashed and lost all my iTunes content.

That isn't normally possible without buying them again. If desired, click here and ask the iTunes Store staff if they'll grant you an exception.

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Audiobooks are a one-time only download from the store. If you don't have a copy of them anywhere (e.g. your backup of your downloads/library, or an iOS device), then if they are still in your country's store and you haven't moved countries since buying them, then you can try contacting iTunes Support and see if they will grant you a redownload : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page

For any other content (e.g. music, films, tv programmes) you might (if they are still in the store) be able to redownload them via the Purchased link under Quick Links on the right-hand side of the iTunes store homepage.