Cannot autorize 2 devices with same account

So I'm trying to autorize both my computer and my iphone so I can have the same book on both device. My computer tells me I can't autorize more than 1 device at the same time, and my Iphone is autorized but not able to download the book since its already used by and unautorize device.... error code is E_LIC_ALREADY_FULFILLED_BY_ANOTHER_USER what should I do ?