1: If you haven't done already download the Kies program from the Samsung site to your PC and install it. Connect your Tab via the USB part of your charging lead to your PC and when your Tab is connected a Phone Icon will appear in the top right corner of the Kies program. Browse for your book files on Kies and drag them to the Phone Icon and they will be copied over to your Tab.
This applies to most other files too ie. music, video etc.
2: Transfer them via a USB microSD card reader to a microSD card that you have previously formatted on your Tab.
It is also possible to transfer them via wifi, but you will need to have a wireless network and your Tab to be shared on the network.......which is a bit more complicated to set