Best Book for Those Raised Without Religion?

A good number of people recommend G.K. Chesterton’s The Everlasting Man.

And if she likes that book, I would then highly recommend Archbishop Fulton Sheen’s A Life of Christ.

For further formation, I’d encourage her reading the Bible in tandem with the Catechism of the Catholic Church and, to better both, informally sitting in on an RCIA classes and adult education classes at a good Catholic parish you can recommend to her.