It will depend on how the books were put on the current tablet. Are they downloaded through a specific app? If so then you would need to have that same app on the new tablet and redownload the books to the new device. If it is either a free account or books you paid for you should be able to download them to your new device once you have logged into the apps system under your original ID with them.
Now if they are specific files either downloaded or installed from another device, you can then copy those items to a SD card and then put the card in the new device to move them over to it, or you can copy all to a computer and then over to the new device (via USB cord).