If you want to delete from device, do as AQ suggests.
If you want to delete from your library (as in permanently delete without a trace and no further access unless you buy the book again), sign into your Amazon account online, drop down your account menu to Manage Your Kindle, drop down the Actions menu on whichever book you want to delete, tap on "Delete from library." Gone for good. (Give it a minute or so to update on Amazon's data base, synch your Kindle app on iPad (tap on the curly arrow on lower right), then there should be no trace on your iPad as well.)