The Xenocide Mission, by Ben Jeapes, marketed as a Young Adult book, and works very well as such, though it is fully enjoyable foradults, in my view. (The lead characters include two humans in their early twenties, as well as a much older human, and an apparently much older alien.)
This novel opens on the joint Human/First Breed satellite called SkySpy, which is monitoring the fearsome aliens known as the Xenocides, or XCs, because they brutally exterminated the other intelligent species in their solar system. Young Joel Gilmore and his First Breed (or "Rustie") partner Boon Round are making an external repair when the XCs mount a surprise attack. The first priority is to ensure destruction of the computer banks and the removal of any chance of the XCs gaining FTL technology. Joel, thrust into a leadership role all of a sudden, heroically returns to the satellite with Boon Round while the rest of the survivors make their escape. Soon Joel and Boon Round are captives of the Xenocides.
One of the key Xenocides is Oomoing, a senior Mother of the Scientific Institute of one of their nations. She is summoned to SkySpy to study Joel and Boon Round. We learn that the Xenocides are fierce carnivores, and very warlike in basic orientation, but also that they are intelligent and not, on the face of it, xenocidal. Also, there is some strange mystery concerning the "Dead World", former home of the race the XCs exterminated. And as Oomoing tries to make peaceful contact with Joel and Boon Round, they get the chance to escape -- but only to the Dead World, and with Oomoing and another XC along.
Meanwhile, a rescue mission is organized from the Roving, home planet of the Commonwealth of Humans and First Breed. Joel Gilmore's father, Michael, a retired Commodore, and Joel's girlfriend, Marine Lieutenant Donna McCallum, in their different ways wangle themselves onto the mission. Once in XC space, the mission takes an unexpected turn, as the Xenocides attack and the Commonwealth ship is thrown into chaos by the treachery of an Earth-based observer and the only survivor of the race that formerly dominated the First Breed. (All this refers to back story which is presumably told in His Majesty's Starship.) Another xenocide is threatened, and so is the potential for cooperation among at least four alien races. Only if Joel, Boon Round, and Oomoing can come to terms with their mutually alien natures and work together to understand the mystery of the Dead World can this danger be averted.
This book is very enjoyable, a brisk, entertaining read, with some pretty neat alien races. It's a bit cynical about politics, both human and alien, but not in a hopeless way, rather a fairly realistic way. It might be too busy, with too many strange alien mysteries revealed, and too many plots within plots uncovered. Still, I liked it. It certainly might appeal to its intended audience of "Young Adults" -- and it will also appeal to adult readers of SF looking for a fine adventure story with some interesting speculation about alien races.