When Apple released iTunes Match it solved a problem: it made music that users didn't buy in the iTunes Store available on any device they had without syncing those devices to iTunes.
It allows users to rip a cd (or download), add it to their iTunes Library and iTunes Match automatically uploads those files and makes them available on iPhones, iPads or other Macs over the air.
With iCloud Drive Apple offers a similar service for your files. You create a document in Pages on your Mac and iCloud Drive syncs it to your iPad and vice versa.
But there's one app that curiously lacks any notion of syncing or storing user created content, and that app is iBooks.
When you buy a book in the iBooks Store it's available on any device and ibooks syncs bookmarks, highlights and reading position.
A device that's got iBooks installed also allows users to store PDF or ePub files in iBooks. Download a manual for your new tv on your iPhone? Safari offers to save it in iBooks. A collegue emails you a report? Mail on your iPad offers to open it in iBooks.
But when you return to your Mac and open iBooks there's no way to access those files. Aside from purchases content each library is a silo agnostic of content on other devices.
Somehow someone at Apple forgot to add syncing to iBooks for those kind of files. And I can kinda understand why in a world where iCloud Drive doesn't exist. But these days? No.
iBooks should have a collection that syncs with iCloud Drive. That collection is then available on any device and syncs any file you save to iBooks across devices. And that same collection should show up as a folder in the Finder. That manual you saved for your TV? Available on your Mac. That report you saved on your iPad? Also available on your Mac. And those PDF's saved in PDF Expert? Well with Document Picker integration even those file can be read in iBooks.
It's just an idea. But I would love to see this in iOS 9.