To register it go to the settings, then there should be a menu named my account or similar. There should be a button to register your kindle there. As for books,open up the books menu from the carousel and switch to the cloud tab. You should just have to long press on the cover and select download to device.
I think you'll find it's pretty straightforward. The thing to make sure of is that it has already been de-registered from the former owner's account and that it's basically empty. If not, when you get it, you can do a factory reset. It should then appear just as it would if new and pretty much walk you through the steps to register and set up wifi and all.
BTW. . .meant to include: to do a factory reset, drag down from the top to display the menu bar, tap more and then device. The bottom option is 'reset to factory defaults'. It will ask you to confirm and then will wipe the device clean.
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