Professionally, it might be The Pragmatic Programmer. It was published around the time that I was becoming a professional programmer. Prior to that I'd only written little one-off horrible disasters to scratch personal itches. It seemed to me at the time that I had begun to form some ideas on what was necessary to be a professional, and The Pragmatic Programmer made all my vague ideas immediately clear to me. I can't speak to its overall greatness, but for me, it was definitely the right book at the time.
-- Ricardo Signes