Here is a list of some essential Christian books that have had a significant impact on Christian theology and spirituality:
"The Bible" - Various authors
"The Confessions" - Augustine of Hippo
"The Imitation of Christ" - Thomas à Kempis
"Mere Christianity" - C.S. Lewis
"The Purpose Driven Life" - Rick Warren
"The Chronicles of Narnia" - C.S. Lewis
"The Screwtape Letters" - C.S. Lewis
"The Case for Christ" - Lee Strobel
"The Divine Conspiracy" - Dallas Willard
"The Cost of Discipleship" - Dietrich Bonhoeffer
This list includes both classic and contemporary works, and covers a range of Christian theology and beliefs. It's worth noting that different people may have different preferences and different understanding of Christianity. This list could be different for others. It's also worth noting that this list is not exhaustive and there are many more great Christian books out there to explore.
I suggest checking online communities, local libraries and bookstores for more options in your area, and ask for recommendations from your local pastor or Christian leaders.