Very good science fiction novel starting with first contact by an Earth empire that's already settled many planets, with a similar alien galactic empire. Emissaries try to negotiate a treaty but on the way back they're lost--their FTL drive sends them to another galaxy. It's sort of episodic, with another plot development in the other galaxy. Reminded me of a couple Star Trek episodes (they also had transmats, like Star Trek's transporters), though it was written a few years earlier than that show; I really wouldn't be surprised if some of Star Trek's writer's had read this novel.
The only flaw I found was in how fast the emissaries were sent after first contact with no preparations whatsoever -- literally the next day.