"The Gods Themselves" by Isaac Asimov is one of the standards of science fiction. My impression of Asimov is that he was a man of intriguing ideas, and a man with a powerful intellect. This book certainly bears that out. Parallel universes, sharing materials, energy, and laws of physics across universal boundaries; alien sex, lunar colonies, and so on. For the most part, I enjoyed the book. Asimov's ideas rate 5 stars. But the book is a little shop-worn with age (published in 1972). It seemed to bog down in lengthy verbal exchanges between characters. The middle third of the book involved the alien society, and that portion particularly dragged. But if you have read and enjoyed Asimov before, you'll certainly want to read this one.