One of the first Voyager books I read. The story is as good as any episode.
The U.S.S. Voyager finds itself in a system where a planet might have existed, but doesn't. Where the planet shoud have been, missions and then billions of people are appearing from nowhere and dying in the vacuum of space.
Voyager answers a distress call to a pre-warp planet that is experiencing strange changes every time their new transporter system functions.
Gradually Voyager discovers that with each pulse, the parallel universes come into contact and the entire population of the planet moves into the next dimension. And in some dimensions the planet has been destroyed, so that billions of people are dying with each pulse.
The book follows the crew of Voyager in our universe and two others as they search for a solution to this problem. This isn't helped by the fact that some of the crew members are in the wrong dimension.