As far as Christian authors go, leaving aside specific theology stuff, I'd go for Corrie Ten Boom's autobiography trilogy. And I can't get past Adrian Plass, coming to the faith through various struggles with mental health and careeer, able to bring me to tears of laughter and tears of empathy in the same chapter. His (slightly) tounge-in-cheek "Sacred Diary" series is a well thumbed source of joy - and theology when you're not looking for it. He manages to show a mirror to the church, and show where we are really getting it wrong. One of my many favourite quotes of his is "Why do I follow Jesus?" "Because He's not as straight laced, boring and judgemental as some who clain to know Him."