Officially I don't think you can. Unless of course they are Kindle books that were free in the first place.
I would imagine that you could insert the external micro sd card into your pc, which would appear as a drive on your My Computer. If you have eBooks on your pc you could drag and drop them onto this drive (best practice would be to create a folder first on the external sd card). Then make sure you eject the sd card from your computer and plug it into the tablet. You can then use an app such as FB Reader to read any eBook that is on your device. Once in FB reader, browse to the external sd card, select the folder you created on it and select an eBook.