Hello axelboy,
Thank you for the question. To read your iBook, you will need the iBooks app on your iPhone. You can download the iBooks app from the App Store on your iPhone:
Where can I find the iBooks app?
You can download the iBooks app for free from the App Store on your iPad, iPhone, or iPod touch.
iBooks: Frequently Asked Questions (FAQ) about iBooks and the iBooks Store
http://support.apple.com/kb/HT4059
Thank you for using Apple Support Communities.
Best,
Sheila M.