From amazon.com:

The Earth has been invaded by the Illyri, a beautiful, civilized, yet ruthless alien race.

Humanity has been conquered, but still it fights the invaders. The Resistance grows stronger, for it is the young people of Earth who battle the Illyri.

Syl Hellais, conceived among the stars, is the oldest alien child on Earth, the first to reach 16 years of age. Her father is one of the rulers of the planet. Her future is assured. And Syl has hidden gifts, powers that even she does yet fully understand.

But all is not as it seems. Secret experiments are being conducted on humans, the Illyri are at war among themselves, and the sinister Nairene Sisterhood has arrived on Earth, hungry for new blood. When Syl helps a pair of young Resistance fighters to escape execution, she finds herself sentenced to death, pursued by her own kind, and risks breaking the greatest taboo of her race by falling in love with a human.

Now the hunter has become the hunted, and the predators become prey.

And as Syl Hellais is about to learn, the real invasion has not yet even begun....

From amazon.com:

Following Conquest, the first part in a stunning new science fiction trilogy that "should not to be missed" (The Guardian), Empire continues the journey of Syl and Paul as they fight to regain planet Earth from a ruthless alien species.

She is the trophy of a civilization at war with itself.

He is its rebel captive.

Separated by millions of light years, they will fight to be united....

Earth has been conquered and occupied. The war is lost.

The resistance still fights the invaders, but they are nothing more than an annoyance to the Illyri, an alien race of superior technology and military strength.

When caught, the young rebels are conscripted. Part soldiers, part hostages, they join the Brigades, sent to fight at the edges of the growing Illyri Empire.

Paul Kerr is one such soldier - torn from his home and his beloved Syl Hellais. She is the first alien child born on Earth, a creature of two worlds - and a being possessed of powers beyond imagining. Now both must endure the terrible exile that Syl's race has deemed just punishment for their love.

But the conquest of Earth is not all it seems.

There is another species involved, known only as the Others, and the Illyri will kill to keep their existence secret.

Light years from Earth and millions of miles apart, Paul and Syl must find a way to reveal the horrifying truth behind the Empire and save all they hold dear from the hunger of the Others.

Even at the cost of their own lives....

From amazon.com:

The third thrilling Chronicles of the Invaders adventure from New York Times best-selling author John Connolly and Jennifer Ridyard mixes classic sci-fi with rich, character-driven plot as Paul and Syl fight to save Earth from an enemy who is closer than ever.

Syl Hellais and Paul Kerr have traveled through Derith, the mysterious wormhole from which no traveler has ever returned. Yet Derith's secrets are darker than they imagined, and, trapped in a dimension beyond their own, they finally emerge to discover a universe that has moved on without them.

Years have passed, and civil war rages among the Illyri. It is whispered that Earth is lost, prey to the alien parasites known as the Others, and other worlds will soon follow. Most shocking of all, the sinister Archmage Syrene of the Nairene Sisterhood has disappeared into the bowels of the Sisterhood's lair.

But before she cloistered herself, Archmage Syrene chose her replacement. The Sisterhood has a new leader with her own plans for the future of her race. Now Syl and Paul, teenagers in a deadly adult world, must find a way to change the course of history and save the lives of billions. They have but one hope.

For Syl Hellais is changing....