Into The Nebula (Book #36)

by Gene DeWeese

Continuing on its exploration of space, the Enterprise and its crew come across a massive gaseous cloud - a Nebula surrounding a sun and it's four orbiting planets. Just after the discovery, they spot a damaged ship. A generation ship with 10,000 lifeforms and various flora and fauna on board.
Ninety years previous, the occupants had fled their slowly dying planet Krantin. 500 years previous, the nebula - or plague as it was know to the Krantinites - began to form, and year by year, the planet's inhabitants changed their way of life in a valiant effort to survive. Some gave up and took to the skies in a convoy of ships. All but one vanished.
An away team beam down to the planet's barely surviving undercover city to investigate and decide that the Nebula is man-made rather than a natural phenomenon.
Suddenly two ships appear from nowhere and vanish just as quickly. It happens several more times, each time giving the Enterprise crew something more to go on. They find that some of the energy pulses that the ships emit also stems from the mines on Krantin, and Data investigates. Much to the consternation of the scientist Zalkan, who Deanna discovers is hiding something.
Eventually, Zalkan reveals that he is from an alternate reality and that the Nebula is a by product from his duplicate planet. They had found a way of ridding their waste, unwittingly destroying the other Krantin. After mining their own planet's Dilithium, they take advantage of the duplicate Krantin's plight and begin to covertly mine their Dilithium.
The duplicate Krantin is run by a vigilantic group called Dictorates, who, threatened by the Enterprise's interference, attempt to take over the crew, but with the dying Zalkan's help, they manage to not only defuse the possibility of war, but get them to agree to help the other Krantin planet and restore it's toxic air.

Notable Imzadi moment:

Pages 193 to 194

As always when he was snatched from a sound sleep, Commander William Riker lurched toward consciousness with shreds of dreams still clinging to him. This time, not surprisingly, they involved Deanna. And Lwaxana, who was seated imperiously in her luxurious home on Betazed in a transplanted captain's chair while all around her swirled the polluted, acrid-scented atmosphere of Krantin. Riker himself, transmuted into the clean-shaven ensign he had once been, stood defiantly before her while Deanna's body appeared and disappeared in the noxious mists that billowed around his feet and he struggled to formulate a satisfactory response to the question Lwaxana seemed to have been asking since the beginning of time: "Will Riker, what have you let happen to my daughter?"
"Commander Riker!" Not Lwaxana's imperious tones, but the anxious voice of Ensign Thompson exploding out of nowhere, sending the distorted images of Betazed swirling into chaos.

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Thank you Carol for putting this together =)