Knowing God - J.I. Packer
Evangelism and the Sovereignty of God – J.I. Packer
Desiring God - John Piper
Holiness, Its Hindrances, Its Root and Fruits - J.C. Ryle
The Pursuit of God – A.W. Tozer
These certainly are not the most influential books ever written, and probably your list would differ from mine. They made my list because I read each of them at difficult times in my life - times when they challenged my Christian development in a significant way.