The charge was a 'proper' charge, not a temporary store holding charge : About payment card authorization holds in the iTunes Store - Apple Support ?
If it was a proper charge and the purchase went through then the book should show on your account's purchase history and you should be able to download it via the Purchased tab in the iBooks app on your phone.
If you can't download it and you've tried the standard troubleshooting (e.g. logging out and back into your account on your phone, disconnecting and reconnecting to your network, closing iBooks via the phone's taskbar and re-opening and retrying, soft-reset of the phone) then you can contact iTunes Support via the 'report a problem' site : http://reportaproblem.apple.com/