Outside of the Bible, Philip Yancey has probably had the most influence on my Christian walk. Whether in his books or magazine articles, Yancey touches the cords that are deep in my soul. Not to mention his editing of the notes in the NIV Student Bible, which remains a favorite of mine.
His book, What’s So Amazing About Grace, woke me to the wonderfulness that is Grace. We all know that the grace of God is free, but, I think we gloss over that and lose the wonder of it. In our day, nothing is free, even when it appears to be so. You may think something is free, but there are always strings attached. Not so with the Grace of God—we could not pay for it, even if we wanted to.
I have a few others of his, notably, The Jesus I Never Knew, Rumors, and The Bible Jesus Read.