Tapping the book should re-initialize the download. You may have to wait for the download to finish before you can delete the book. Yes, I know, irritating; but iBooks probably has to know the book is really there before it can make it really gone.
Hold in mind that a book viewed in the Purchased tab may not actually be on the iPad. If there is a cloud symbol next to it, then what you are seeing is Apple telling you that you own the book and can re-download it again, for free, anytime you want. It is not taking up space on your device. Well, no more than needed to display the icon and description text.
Short of contacting Apple and asking them to permanently remove the book from your account, there is no way to actually delete a book from your Purchased tab. You bought it. Apple considers that it is yours, and that you should be able to find and download it again.
You can hide books from your purchased tab using iTunes on the computer (or the new iBooks app if you own a Mac). This does not delete the book, it just hides it. You can un-hide apps, books, etc. by accessing your Apple ID account though one of the online stores.
If tapping the book to re-start the download does not work, try restarting your iPad (turn it completely off, not just sleep) and open iBooks again.
Good luck.