Human/Alien Alliance consists of many species including one that seemed to use physical movements(ear position and posture) to partially communicate, they seemed to be a very Spartan like society and had trouble understanding human emotion and sarcasm. Another were a historian or librarian type of species that had a caste system. I believe there were other species as well, including one that had incredible linguistic adaptability to understand other races languages.
The antagonist or hostile alien race were compared to tall multi legged giraffe looking beings that themselves were fractured/divided into several different "fleets" or groups all with the purpose of achieving some sort of perfect symphony or song. They had some sort of telepathic or sound based communication system. They had been systematically wiping out all other races and civilizations throughout the galaxy for quite some time and had at least one slave race that had been either genetically or trained over time to rely on their masters "song". Any distraction or disruption of the song/harmony weakens their capacity to communicate with other ships and their tactical advantages.
The hostile race had apparently been beaten back at a battle near or on earth , after the struggling alliance had found the humans. The victory giving the alliance time to search for allies. It seemed obvious this was a later, potentially not even the second book in the series.
The start of the book began with a fleet made of up alien and newer human made ships manned by both but led by an overall human female mission director. Their mission was to search different parts or arms of the galaxy for other alien races that might have survived the hostile race's holocaust.
In the first battle of the book, the alliance fleet finds some enemy ships positioned over a planet that had for quite some time been wiped out but now colonized by the hostile race. The Enemy commanders were a mated pair that both had some time of musical names like 7th octave or something similar. The alliance used some sort of heavily armed ship to ram into and board the enemy flagship.After the male enemy leader was killed, the Female lost control after losing her mate and the disruption to the local harmony/symphony and attacked the boarders with blades of some sort but was killed.
After having defeated the hostile aliens fleets, they investigate the ruins of the planet's extinct species and find a very horrid and chilling scene where there are frozen/petrified members including children of the wiped out natives floating on the surface of the planets moon or possibly the planet itself having lost its atmosphere.
The alliance had been able to capture many members of the slave race the antagonist race used on their ships. And it was incredibly difficult to begin communicating with them. They appeared to be a reptilian/snake like species that now without their masters constant presence were panicking or entered a sorta of catatonic state. They gathered together in their holding cells in writhing moving mass of limbs and bodies trying to seek comfort and having been captured.
A significant technology of the book was using some sort of plasma shielding to warp/jump in to systems and hiding in their local suns to mask positions and movement.
The Protagonist is the Female Director of the mission, though there seemed to oft be multiple view points. Her position as leader of the operation is rare and new and she seemed very aware and partially self conscious of that fact. Humanity was relatively a new member and so still needed to prove itself. A significant decision she needed to make at the start of the book was whether to return their fleet back to Earth or continue jumping/warping farther in the chance of finding more allies.
-The book was found in 2017, at a public library.