This is a great novel by Asimov, and it won both Hugo and Nebula awards. It was also Asimov's favorite of his Science Fiction novels.

The book is made of up three different sections. In the first section, a new power source is discovered that works by exchanging material with another parallel universe. At first, it seems miraculous, but eventually a few people realize it might actually destroy us. But try to get people to give up cheap, limitless energy! By exchanging messages with the other parallel universe, the earth scientist who understands the danger tries to get the people in the parallel universe to stop the exchange.

The second section takes place in the parallel universe, and here Asimov was truly creative. The race he comes up with has three sexes, and he works out how they relate to each other in producing offspring.

The final section is back in our universe, where a solution is finally found.