This would have to be Tim Keller's "Prodigal God".
In 2014 I've been very privileged to be able to read some excellent Christian books, as well as some that were not so good and one, I think, that was frankly dreadful.
Why choose Prodigal God as my favourite?
It's not just because Tim Keller lives in New York, likes jazz and is one of the Gospel Coalition Leaders. (I didn't like his book on Judges very much)
It's because, just when you thought it would be impossible to say something new and fresh on the parable of the prodigal son, Keller arrives with a view so full of grace and Christ and the gospel that it became immediately my favourite book to give to any Christian who hasn't read it, especially if they might not have quite grasped the gospel.
And it did me good. And I think that each time I read it with someone it will do me more good. It's that good.