As the colons indicate, this is the first book of the sequel series to the Lost Fleet series, which I reviewed here. It will have spoilers for the first series.
John Geary, once again Admiral, and Tanya Desjani return from their all-too-brief honeymoon to find a foolish order subjecting many officers of the fleet to courtmartial on a technicality, and the fleet itself on the verge of mutiny. Geary manages to persuade the governmental officials to rescind the orders, but it leads into more troubles as he finds himself ordered to go forth and find out what can be found out about the aliens who caused so much trouble in the last series, and the century before that. And learns some, and guesses more, about governmental intentions.
This requires going through Syndicate space, or what was Syndicate space, and having some convolutions there, too, but they make to the space where the enigmas live, and start to make discoveries.
This weaves on to the end through hollowed-out asteroids, fights, rescues, attempts at blackmail, romantic conflicts, advice to punch someone in the stomach if he tries to bring up a topic again -- and mysteries. Lots of mysteries, and some clues to them This is the opening of a new series, though at the end they have reached a definite transition point.