How do I delete books from iTunes?

I have a bunch -- a whole bunch -- of sample books and read-and-done-with books in my iTunes library that I've downloaded on my iPhone and iPad. I'm finished with them and would like to remove them from my iDevices and iTunes library.

I have deleted them from my iPhone, my iPad, and iTunes (deleted and moved them to the trash), only to have them reappear the next time I sync. I understand that they are permanently listed in my Purchases list, but why do the book files keep reappearing on my iPhone/iPad/iTunes library? How can I permanently remove them from the iTunes library?

Thanks!