Second Best

Like Carl, I have a warm appreciation for Machen's Christianity and Liberalism.  On a personal and biographical note, the book had a very important influence on my father's theology and discipleship when he was in college and trying to sort out his own beliefs about Scripture.

I would demur, though, from calling it the second most important book you'll ever read.  For me, the second most important book will always be a hymnal, preferably the Trinity Hymnal.