There's a known problem being investigated by the development team that when you attempt to download an eBook you may be requested to login even though you're already logged in, or you may be asked to register again to download the eBook.
While you should be asked to confirm your details, and these should be automatically populated, you shouldn't be requested to re-register or re-login to download an eBook.
The development team are currently investigating this and there's currently not a timeline for a fix.