If you have them on your computer's iTunes then you can re-sync them to your device (are they in the Books part of your computer's iTunes library ?) - that has not changed, only the layout of iTunes has changed with the left-hand sidebar not automatically showing, though you can enable it (option-command-S on a Mac, control-S on a PC)
If you don't have a copy of them on your computer nor on a backup then you will need to contact iTunes support and see if they will grant you a re-download - as far as I know it's never been possible to re-download aubiobooks from the store without contacting iTunes support.