I'd recommend this be one of the first books you read as you start your spiritual journey. This is a profound book that will jolt you awake from your apathetic musings and stir you to the depths of your soul.
It's one of those books (like E. L. Prentiss's "Stepping Heavenward") that feels like it was written JUST for you. "Screwtape Letters" has that same feel - that C. S. Lewis crawled into your consciousness and described every mental battle you've ever had - and explains that those subtle arguments which steered you away from spiritual growth, were cleverly disguised devilish whispers. As Lewis points out, the path to hell is a gentle slope.