Recommended Readings List:
While the quick answer to what every Christian should have read is...THE BIBLE... there are a lot of good supplemental resources that God's people have produced as they have reflected on God and their own spiritual journeys. I'm going to add to this list and possibly create different categories, but let me start with a list of "twelve essentials."
Pilgrim’s Progress – John Bunyan
Mere Christianity – CS Lewis
The Knowledge of the Holy – AW Tozer
Knowing God – JI Packer
Prodigal God – Tim Keller
The Return of the Prodigal – Henri Nouwen
Desiring God – John Piper
Spirit of the Disciplines – Dallas Willard
Confessions – Augustine (this can be a hard read!)
The Lion, The Witch, and the Wardrobe – CS Lewis
In the Grip of Grace – Max Lucado
The Cost of Discipleship – Dietrich Bonhoeffer