It depends on which model you have. On the new one, you can highlight the title on your home screen, then click the left arrow on the 5 way selector button. It'll ask if you want to delete the title, click the center of the button and away it goes.

I can't remember how to delete from the 2nd model. The first, you have to plug it in to a computer, and (I think) just open the kindle folder. It's been a while since I used that one.

In any case, I'm sure a quick google will bring up the answers.