I read this book about 25-Sep-2003. This is the first time I've read this book. The book is copyright 1987. This note was last modified Monday, 05-May-2014 22:13:53 PDT.
This is book 3 of the "Trigon Disunity" series.
This note contains spoilers for the book.
We never do find out just what the Mizari are. But it looks like they won't be coming back and sterilizing Earth again soon.
But we have no explanation at all why they did it the first time, either. They do destroy several human ships. The don't try to go after their sources. It's vaguely suggested that they went after the ships only when they weren't transmitting (little hint of Ventus here), and the state of the systems suggests a massive cleanup; but that doesn't explain why they followed up the starship of the Founder civilization and sterilized Earth, but apparently make no attempt to follow up on the more recent contacts once they've destroyed the ships.
There's also a big part played by the clash of some crazy people who can't feel safe knowing the Mizari are out there (this is crazy?) and some other crazy people who think that it's best not to disturb them. I side with the first people except that it's so obvious they're nowhere near up to the job; so that the second course is the only possible logical one at this point. Unfortunately I don't think anything has been added to my understanding of this class of conflict.
Merritt Thackery, the twit from book two, is back, and in the end he saves the day by destroying two human battle-groups magically from out of the blue. Then he gets to go off and meet Gabriel again, which is certainly a good outcome from his point of view.