I've amassed a huge pile of spent spellbooks (known spells that have been read and/or polypiled into uselessness), and know almost all the spells (still trying for Finger of Death). I've yet to get a marker, so I can't write any books to refresh many of my known spells. So I was wondering if I use Cancellation on the books prior to polypiling them, will they become readable again?
After testing a dozen or so times, it appears the answer is NO. So, polypiling spent spellbooks can only be used to attempt to get unknown/unidentified spellbooks. However, if you need several spells, you can write a low-level one, read it, and get 3 more poly attempts before it is spent, too.