These are the books by Anthony Horowitz that are reviewed on this site:
Alex Rider's back! 'The nearest thing the Brits have to a lethal weapon!'
Reluctant teenage spy, Alex Rider, meets arch-villain, Herod Sayle.
Matt is the first of The Five to be found. He has to find the other four. He's in a battle against the Old Ones. It's a race against time. And he doesn't even know it yet.
If Matt really is the first of the five, how can he find the others? And what are they supposed to do? There's a tight deadline here, to save the world.
The Old Ones have escaped from their prison and are bringing great evil and destruction to the world. Only the Five can stop them, but the Five are lost and in disarray.