Can I tell who used iTunes on my iMac to sync while I was away?
One way to tell if a device has connected to your iTunes is to check the preferences and see if there is a device backup.  When you can connect a device to iTunes it creates a backup of the device (if you are syncing stuff to and from it).  In your case it sounds like you wanna find out if your books have been syncing to another device.

Open iTunes Preferences:


Windows: Choose Edit > Preferences
Mac: Choose iTunes > Preferences


There will be no way for you to see what specifically was sycned, this will only tell you if a device has synced, and when.