Axis is set 30 years after the end of the first book, and sheds a bit more light on the nature of the Hypotheticals, the mysterious entities that hid the Earth away from the normal passage of time. Not much more, mind you, but then the point of the books is to explore how mankind would react to direct contact with something entirely unknowable. As before, the characters have a depth and authenticity that stands out. The story is definitely about the people caught up in the events, not the events themselves.
A good read. Highly recommended if you've read "Spin".