Thanks for the input, everyone. I actually tried to recommend Carl Sagan and his work but she said that he is hard to understand. Not sure why..

The Demon-Haunted World.
The Skeptic's Annotated Bible
God is Not Great.

My favorite (non book) that really explains the process of transitioning from theist to atheist and what it is like being an atheist is worth the watch - - for me, there were parts that were literally hair raising. Worth watching even if you are an atheist. If you are a theist, I recall a story about how a woman on the fence made the transition from superstition to reality by watching this series. .

The Koran.

It might seem unrelated because it's about cave paintings, but The Mind in the Cave by David Lewis-Williams sneaks in a bit about early religion and how it eventually led to creating modern McGod church franchises. Very interesting book!

Origin of the Species - Charles Darwin.

What started pushing me away from Christianity was Heinlein's Stranger in a Strange Land.

The Christian Delusion has a lot of good arguments from different authors. You never know which perspective is going to change a mind.

The End of Faith by Sam Harris was something that helped me to let go. I’d already started questioning and left the religion of my childhood but was clinging to the need for ritual. This book highlights the role organized religion plays in so many of the horrors we see today.

Charriots of the gods is a good one!

If she is a christian then she already has the book! The bible is full of stories and if she really thinks about them she will note that they are really untrue! Man can not live in a Whale and where did Adam and Eve's sons find wives are just a couple of the stories and the book is full of them!

Why don't you just talk with her? Invite her to dinner and ask each other questions about religion and atheism.

Sense and Goodness without God, Richard Carrier