This is because you chose the option to sync your iBooks with iCloud. This deletes everything with the exception of iBooks purchased from the apple store. Apparently Apple knows best what should or should not be on your computer and your devices.
You will also find that on your devices, any iBooks that are not purchased through the iBooks store will need to be downloaded all over again (click on unhide iCloud books--they will have the cloud with the downward blue arrow). I had over 8.5 GB of iBooks this occurred with. Irritated is hardly the word I would use about this.
You have two options:
1.) keep your iBooks in the cloud and when you want to read one of your books on your device, download it from the cloud all over again. If the book was not purchased from the iBooks store, it will no longer show up in iTunes on resync. (At least that is what I found to be the case on my computer). This is especially annoying because I don't always have internet access... and that Apple made the command decision to delete all 8.5 GB of data from my devices really concerns me. They should have at least provided the option for ME to choose what to do... or clearly indicated that this would happen if I chose to sync to the cloud. They did neither. I'm highly irritated about this and wonder what their engineers are thinking!
2.) Do the following on your computer if you have a back-up on something like Time Capsule or another drive:
1.) Go to System Preferences on your computer and open iCloud. Click on "Options" next to iCloud Drive and deselect iBooks. This will prevent iBooks from syncing your iBooks to iCloud.
2.) Go to Users/Library/Containers and find the folder com.apple.BKAgentService and the folder com.apple.iBooksX. Copy these two folders to your desktop to back them up in case something goes wrong.
3.) Restore from your backup the above two folders ("com.apple.BKAgentService" and the folder "com.apple.iBooksX").
4.) Open iBooks. Sign out of your account and sign back in. Your iBooks should be back on your computer and resynced through iTunes.
If you get an error message when trying to open a book that it can't be found, or all your books begin to disappear again when reopening iBooks on your computer, wait and let the app finish what it is doing. I found that it stalled my computer (and my internet connection out for some time), but it deleted and then restored the iBooks, but they were not in the manual order I had originally stored them in my collections. To fix this, I closed the program and copied the folder com.appl.iBooksX over again. When I reopened the app, everything was in place again. The folder "com.apple.BKAgentService" contains your iBooks while the folder "com.apple.iBooksX" apparently contains the information necessary for iBooks to know what books and in what order in the collections they appear. They will also reappear in iTunes.
I had to then complete an additional step as I had synced my devices through iTunes which resulted in all 8.5 GB of my iBooks being deleted as Apple thought I should download them all individually again from my iCloud. I had to restore my devices from a back up. This could prove tricky if you have iTunes set to automatically sync when your device is plugged in. If that is the case, you should check the option in iTunes preferences to stop automatic syncing when the device is plugged in. Also from a back up, find your MobileSync folder in your User/Library folder and copy those over to its equivalent on your hard drive (making sure that you back up the original one that was there in case something goes awry). Then, restore your device from the copied over back-up. The reason to do this is because if you updated to OS 9.3, then it would have replaced your original backup of your device with the one where you chose to sync to the cloud. Once you have restored your device, your books should copy over again and everything be back to normal.
Hope this helps. Sorry it is so long :-)