I agree with Philip Goldberg that it is quite interesting to explore what people really mean when they use the word God. However, I'm not as convinced that "the G-word has undergone a radical change in recent decades." I think theists and atheists alike have always used the term in varied ways. He references a few sources that are well worth reading and even provides links to them. There are a wide range of materials that are excellent for spurring further thought on the concept of God. At this point I'd like to recommend two books in particular.
Andrew Pessin's The God Question provides a wonderful overview. I read it a few years ago and have re-read it a few times since. The only thing that would have made it better was if he had added in a few choice excerpts from the works he references.
Froese and Bader's America's Four Gods provides a very readable and fascinating analysis. Personally, I would divide the God concept into two fairly broad versions: concrete and abstract. I tended to think of their four as fairly distinct variations that contain elements of both but with each leaning towards one or the other.
Whether you believe or not the concept of God with all its variations and implications is an interesting topic of study.