If I remember correctly, Rabbi Avigdor Miller ZTL ZYA
once said, in a public lecture, that if you want to find
kosher secular books to read, then search for books
that were written a very long time go,
before the secular society became atheist and immoral.
If you want kosher secular books to read, then I suggest
the Sherlock Holmes books by Sir Arthur Conan Doyle
and “The Hobbit” and “The Lord of the Rings” by JRR Tolkien.
Also biographies of people who lived long ago,
like Benjamin Franklin and Thomas Jefferson.