Welcome to MobileRead, nhenry!
To move books into and out of internal memory, you have to use the Connect Software, or, one of the alternative, homebrew apps that fill the same function. Such as LibPRS500
, though I really think that one is too good an app for the description "homebrew."
If you want to avoid using one of those for some reason, you can use an SD card to read books. But it won't get the old stuff out of the internal memory, and it does have a bit of an increased battery usage to read from SD cards.