iBooks disappearing on iphone after sync

This recently started happening, and I can't for the life of me figure out how to resolve it.

Whenever I sync my iBooks library to my iOS devices (iPad & iPhone), as soon as I open iBooks on my iPhone, I can see the books, but then that little cursor starts spinning and afterwards 75% of my collection is just gone! This happens every time. I only have 420 books in my collection, so I started fresh, deleted all of them, got rid of iBooks for OS X, imported them back into iTunes and spent 15 hours [total] going through every book and making sure that the tags and filenames and artwork were all the same resolution, everything! Deleted the books off the iPhone and synced and held my breath. Well, they sync, but again, shortly afterward, most of them disappear. On top of this, the sync preference automatically changes to sync some books instead of all books without my changing it. I should also mention that I tried it without the setting on the iOS devices to sync collections or bookmarks, etc., but made no difference. AND I have plenty of space remaining, more than 10GB, and the books are a total of ~650MB.

Even more vexing is that the iPad syncs perfectly every time. I can definitely trace this to happening after the iOS 8.4 update, before then it never happened.

Sorry for the lengthy post, but the more information up front, the fewer questions asking me if I tried something that I had already done.

Anyone have any ideas?