I believe a simple iTunes sync should solve the problem. Just sync the iPad, and then sync the iPhone. You might also be able to just buy the book again, but they won't charge you because they know you already own it.
You don't need to sync with itunes to get the book from say and ipad to an iphone. If you go to iBooks from your iPhone, and you tap "store" then tap "purchases" in the lower right corner it will show you all of your iBooks and you will have the option to "redownload" listed next to books that aren't currently on the device.