Tony Kan from My Apple Newton shared his page of free Newton eBooks available for download.
Kan converted many books (“an eclectic mix of spy and war thrillers, science fiction, fantasy and classics”) to the Newton’s easy-to-use eBook format:
Some of them are in the public domain and others are books that I own hardcopies of but they have been scanned so that I can read them conveniently on my Newton…However, be mindful that you should only download and keep the books that are still under copyright if you have already purchased a hardcopy in the past.