Two books for me that were majorly impactful were:
That All Shall be Saved by David Bentley Hart. This book deals a devastating blow to the idea that hell is eternal and inescapable, in contrast to what Catholicism teaches.
How Jesus Became God by Bart Ehrman. This book introduced me to the world of biblical scholarship and provided a second opinion on how Christianity got started.
Popular alternatives to
That All Shall Be Saved: He...
Did Jesus Exist?: The Historical Argument for Jesus of Nazareth