If you’ve bought a book in iBooks on your iPad, when you open up iBooks on your iPhone, it will not be there. Unlike Amazon, Apple does not automatically sync these libraries. It is kind of a clumsy process, but easy enough to manage. Essentially, you have to manually redownload every book that you have purchased from iBooks again. But thankfully, you don’t have to buy them again. The download is free.
Open iBooks on the device that you did not purchase the eBook on. Click on the “Store” button and you should go into the iBooks store. On the bottom right hand corner of the screen is an icon that says “Purchased.” (It’s right next to the “Search” icon). Click on this and you should see a list of books that you have purchased through your iBooks account. Click on the book, enter your Apple ID when prompted, and the book will show up on your digital bookshelf. Wash, rinse, repeat.
Remember, it has to be the same iBooks account, so if you and your spouse are sharing an iPad, be sure to use the correct login to access your account.