[Corpora-List] Work of literature without copyright issues

Dear Colleagues We are currently working on manually translating domain-specific English data to multiple low-resource languages. Literature is one such selected domain. We started with novels in project Gutenberg. However, most of those books are from the 1800's and our translators struggle to comprehend the language used in those books. I would like to know whether there are story books written in modern simple English that are free of copyrights to be used for this task.

Regards Surangika