"After book (1) below, it is an impossible question for me to answer exactly. Many books have influenced me at different times. Here are eleven that influenced me quite early in my Christian life (all but #3 and #10 and #11 while I was still in college, before I went to seminary), plus #12, which influenced me many years later.
(1) The Bible, far beyond all other books combined.I've read most of them myself! Pretty chilling for a non-Calvinist.
(2) John Calvin, Institutes of the Christian Religion
(3) Louis Berkhof, Systematic Theology
(4) J. Gresham Machen, Christianity and Liberalism
(5) Cornelius VanTil, The Defense of the Faith
(6) D. M. McIntyre, The Hidden Life of Prayer
(7) John Murray, Principles of Conduct
(8) John Murray, Redemption: Accomplished and Applied
(9) B. B. Warfield, The Plan of Salvation
(10) B. B. Warfield, The Inspiration and Authority of the Bible
(11) Geerhardus Vos, Biblical Theology
(12) John Wimber, Power Evangelism"
Calvin is a great read: his brilliance shines through, even though his Stoicism does as well.
Berkhof, Machen and VanTil also mainstays of early to mid C20 calvinistic protestantism, but still good education for a young Christian; although I find VanTil's apologetic enterprise obscure, to say the least.
I've not read (7), and may have read (8). The title and author very familiar from my Capernwray days. I've also not read (12) and have no plans to.
The most stimulating of all of them (saving (1), of course) was Vos. Difficult and intense, and maybe that's why.
The great frustration was that as I was reading these books I had no one with whom to discuss them, not even the minister of my church was suitable (no, not you Bob, RK).
Some years later I and a pal read and discussed Thielicke's Evangelical Faith, at least for a few chapters, and that was truly wonderful experience.