John Piper has had a profound influence on my Christian walk. Not because he is great in and of himself, but because he has pointed me to Christ. His life of vigorous biblical study and devotion comes through in what he writes. I'm thankful to God for him and the sacrifices he has made to help us see Christ-centered truth in deeper ways.
I'm currently reading his 2002 book "Counted as Righteous in Christ" . Which is a detailed argument for the doctrine of Justification by faith alone, and the credited righteousness of Christ to us through his death on the cross.
I highly recommend it!