The second book in the Beyond the Frontier series (first reviewed here). Still with The Lost Fleet emblazoned on the cover, even though it had ceased to be lost by the fifth book of the first series. However, you learn to live with marketing departments. And their covers -- this one has a jab at the accuracy of the pictures on the covers of the whole series.
But that, of course, is not the point of the story which is exploration of space and high grade military SF.
Spoilers ahead for the earlier books. (I do not recommend tackling it first. Like most good series, all the later ones will spoil the earlier ones.)
It opens mid-battle, from the cliffhanger ending of the last. They fight a frantic battle against these new aliens and having beaten them off, learn what they can. It seems that this time they have happened on a race of herbivores that look like teddy bears and form massive herds, who have turned their entire planet into uniform homes with gardens on the roof. They succeed in breaking into their video communications, but they can not establish communication with them.
Escaping that system brings them to another discovery. And another battle, with Campbell's usual flair for tactics and apt hand at remembering how great the distances are in space and relativistic effects at even appreciable fractions of c.
The story weaves on through aliens, communications, the course of true love not running smooth for more than one pair of lovers, political intrigues in the Alliance though no part of it takes place there and they get no messages, rebellious planets, and renegotiating bargains. A great story.