The Tolkien estate has never sold the rights to the book, so Amazon’s series (just like Jackson’s films) is based on the Lord of the Rings novels, The Hobbit, and their appendices – that’s it.
A key component is the casting of Miranda Otto, Eowyn, Shield Maider of Rohan from Jackson’s films and here the narrator putting this “origin” story (but really a stand-alone) together for us.