To delete an audiobook's files to free up space on your device, tap on your audiobook. Under the right side of the Play audiobook button you can select Options and then tap Delete audio files.
You will still be able to re-download your audiobook at any time as the audiobook will remain visible in your folders.
If you would like to fully remove an audiobook from visibility within your library, you can now archive the audiobook. To archive an audiobook, tap on your audiobook. At the bottom of the page select Archive audiobook.
Archived audiobooks are hidden from the folders, tags and genres in your library. You can view and unarchive audiobooks by scrolling to the bottom of your library screen and selecting “Archived Audiobooks.”