Does Apple keep files of all photo book orders?

No, only for 30 days. You'll have to reorder the book from your iPhoto Library as you did originally.

Just a note about future books. I create new libraries for each book I create and transfer the photos I need for them from my working library to the book library with the paid version of iPhoto Library Manager. When done the library is usually small enough to be copied to a CD or DVD disk for archival storage. It an be revived by copying back from the disk to the desktop and opened with iPhoto for editing and ordering additional books at a later date.

Yes for 30 days - after that you have to resend the file

And there is no option to purchase just a dust cover - only a complete bok - may be able to have a loal printer print a dust cove from the Preview PDF -

Before ordering your book preview it using this method - http://support.apple.com/kb/HT1040 - and save the resulting PDF for reference - the delivered book will match it.

LN