1. To read more about the brain, coginitive science, and how we think about religion: The God Instinct.
2. To think more about the basic question, "Is There a God?" we could read Karen Armstrong's The Case for God.
3. For a good overall look at what Christians believe, restated for a contemporary audience, there is To Begin at the Beginning, by UCC pastor Martin Copenhaver.
4. To talk not so much about what Christians have believed about God, but what they think they should be doing about it: Jim Wallis' Rediscovering Values (this from a Christian writer) or Peter Singer's The Life You Can Save (from a not so religious point of view).
Here's a link to a "To the Best of Our Knowledge" broadcast on religion in a secular age -- mentioning several books that might be of interest. Thanks to Mick for sending this along.
Here are links to the above books: