The Reason for God is one of the best books I have read on why Christian belief is true belief. It is an engaging essay on how and why a clear-thinking sceptic can take the Christian worldview seriously. It forces you to think about what you think about the world, not just what you don't agree with.
Keller's thesis is that no-one is a pure sceptic. Everyone believes things about the world and people and God. He believes that compared to the alternatives, Christian belief is the closest to the truth about things. Combined with this, he argues that everyone knows God exists, even if they don't admit it to themselves.
The first half of the book addresses common objections to Christian belief. The second half argues for the Christian worldview. There is an intermission halfway through which briefly considers other issues, like why beliefs differ between Christians and Christian denominations. The final chapter explains the implications of his argument for readers.
When I say this is one of the best books I have read, that's because it crosses boundaries in the same way that our own experience does. It has philosophical clarity, it asks us to consider our own experience, it looks to literature and art and science and the world to make things clearer for us.
If you have specific issues, such as questions about a particular philosophical argument, there are other, more comprehensive works dedicated to such things. But for most people this is the most competent overview of all the issues.
Like Mere Christianity by C.S Lewis (somewhat dated now), I would recommend it to sceptics for the reasons above, but also to Christians as an example of how to communicate what they believe clearly and compassionately.