Why is iBooks not syncing?

I have an iPad 1 and an iPhone 4S. I very much enjoy reading books and higlighting them on my iPad. When I open iBooks on my iPhone 4S, there are no books on the "bookshelf" at all. I can go to the store and download already purchased books on my iPhone, but I want iBooks on my iPhone to syncronize my library that I have on my iPad. I want to be able to start reading on my iPhone and resume at the same place on my iPad.

I have both "Sync Bookmarks" and "Sync Collections" turned "on" in my Settings, and I've tried turning them both off, turning only one off, etc. and turning them back on, but iBooks on my iPhone 4S IS NOT syncing with my iPad, and I find this kind of frustrating.

What do I do? I just want to start reading on my iPad and resume reading on my iPhone, like advertised. 😟