Please forgive me if this post is inappropriate, however this is a question that has (imo) remained unanswered for a long time, and (judging by the responses over the last two years), to which their seems no real consensus on a book that really gets this job done well.
As such, I'm panning on writing a book specifically designed for the purpose of helping people understand the spectrum of belief, and where they are on it, and how they can (logically) move along it.
In an effort to bring this book to the world, I recently started a RocketHub Project, (similar to kickstarter, but available in Canada), and I would be grateful for you to take a few minutes to review it, and if you feel so inclined, support it, or better yet. . . promote it via your facebook, twitter, mailing list, or blog post.
The link to the project is here:http://rkthb.co/7212
The reason this book is so important is because it often takes religious people several years, and multiple exposers to people questioning their faith to move them significantly away from it, and unfortunately, despite their compelling arguments, most of the (popular) 'new atheist' books written in recent years, (Dawkin, Dennet, Hitchens, and Harris), are just a little to blunt, and antagonistic to be convincing to those with deep faith. (From the titles alone a theist might get the impression that they are deluded, under a spell, their religion is poisonous, and we must put an end to faith), and yet religious people don't have any of these feelings as their starting point, so something must be done to bridge this gap, and reach people, using the tools of logic they already know and use.
I do hope this post and link isn't removed, and that those reading it, will take the time to support me and promote this important project.
I'd like one day for everyone to be able to point to my book, as the #1 'go to' recommended choice to the question the initial poster asked, and for anyone trying to share the idea of a non-religious worldview with someone they care about. Hopefully, it will also become the best resource for those wanting to have these discussions with their friends, and family, which make a significant difference, and actually progress these types of conversations along to their (hopefully) forgone conclusion.
With your help, (sometime in Dec, 2012), the answer to the question will be. . . "Have you checked out Mark MacLean's book. . . "The Greatest Gift Of All"?
P.S. I HUGE thank you to whywontgodhealamputees for the site. I've sent dozens and dozens of people to view the videos over the years, and many of my closest friends and family members now use these examples (and gambits) in their conversations with believers.