Sync with itunes, or transfare purchases and itunes will transfere all of your data from your iphone. But be carefull to autorize your account on itunes. If you not autorize it will delete all datas from your iphone. Another way is if you can download "iexplorer" to your pc and you can transfere datas like pdf's from ibooks to your pc and then you can send tem via email. Hope this will help you...;)
And if EN really wants to be taken seriously as a PDF development company, they can look into this for what I'm sure is a well under-utilized feature. I'm not really sure what else would be useful on a developer's list other than this.