Underline the book you want to remove on your kindle.
Press the little right arrow on the square button.
It will then present you with "Remove from Device."
Underline that and press the square. It's gone.
To clarify. . . . . if you press the right arrow, you get a whole menu -- the bottom option is 'remove from device'. You have to arrow down to select that option and press the center button. That will remove it.
If it's on the home page, not showing in a collection, you can do it with a few less clicks by using the left arrow. If you do that 'remove from device' will come up right there and you just click and you're done.
If it's in a collection, the option that will appear will be 'remove from collection'. If that's the only collection it's in, you'll now see it on your home page and can delete it from there.
If you set the sorting to show by something other than collections, I think the 'remove from device' will come up on any book shown.