You should be able to rename them with distinctive names. It works on my version of iBooks, and I have an older iPad from 2013. The latest iOS I could upgrade it to was 9.something. You may have to use the Select control (top right of your iBooks screen) first on the book title. Then you should be able to click on the title and change the name. I would assume this is still possible on later versions of the app and iOS.