Deregistration now working.  Add book retrieval progress checks when new books are enqueued.  Works now like Blio, except that all chechecks and enqueueing happens only after all info has been retrieved for each book, because the full BookInfo array including isbns is not available until then (whereas in Blio it is available after BookVault contentsWithTokenEx is complete, and before the ContentCafe metadata calls for each book are complete).