This book is so good that my precious copy was stolen. The authors were the first to see what most searchers for extra-terrestrial life have still not understood: that life does not have to depend on water and carbon. They define life as 'the activity of a biosphere... a highly ordered sytem of matter and energy characterized by complex cycles that maintain or gradually increase the order of the system...' They then go on to imagine all the weird and wonderful systems that might satisfy this definition. They suppose that there could be biospheres not only on planets with oceans of methane or ammonia, but also on the surface of neutron stars, in the envelopes of sun-like stars and even in clouds of interstellar gas. The result is a mind-blowing tour of the universe that reads like science-fiction without ever letting go of accepted science.