I had to think long and hard about this question because no one book immediately jumped into my mind. I've read so many books that have made impressions on me as a person, and as a writer. But really, when I got to thinking, I knew the answer.
The first book I remember giving me some sort of "aha" moment was Piercing the Darkness by Frank Peretti. I was a teenager when I first read it, and this novel was my first introduction to Christian fiction. (I accidentally read it before This Present Darkness.) I was hooked.
The character Sally Beth Roe had the most impact. Especially her conversion. I don't remember if I consciously thought, "I want to write like that." But gradually I realized this as I began to pursue writing. I've always loved her character, and now as I think on what I want to include in my fiction, I remember how unashamed Frank Peretti was about including the gospel in his stories. A lot of authors water down the message of truth these days to make their novels more accessible to the masses. But Peretti reminds me that you can please the masses (can anyone say millions sold?), and speak the truth.