If You Like Anthony Horowitz's
Alex Rider Series, Try These...
Fiction / Teen Fiction

Fiction
Crilley, Mark.  Akiko series.
Dale, Anna.  Dawn Undercover.
Fisher, Cyrus.  The Avion My Uncle Flew.
Frederick, Heather Vogel.  The Black Paw.
Korman, Gordon.  On the Run series.
Lerangis, Peter.  The Code.

Teen Fiction
Anderson, M. T.  Whales on Stilts.
Bradley, Kimberly Brubaker.  For Freedom: The Story of a French Spy.
Card, Orson Scott.  Ender's Game.
Colfer, Eoin.  The Supernaturalist.
Cooney, Caroline B.  Code Orange.
Cooney, Caroline B.  The Terrorist.
Cottrell Boyce, Frank.  Millions.
Craig, Joe.  Jimmy Coates, Assassin?
Cross, Gillian.  Phoning a Dead Man.
Farmer, Nancy.  The House of the Scorpion.
Lyon, George Ella.  Sonny's House of Spies.
MacHale, D. J.  Pendragon series.
Nickerson, Sara.  How to Disappear Completely and Never Be Found.
Haddix, Margaret Petersen.  Among the Hidden (The Shadow Children) series.
Oppel, Kenneth.  Airborn.
Rose, Malcolm.  Luke Harding, Forensic Investigator series.
Shan, Darren.  Cirque du Freak series.
Shusterman, Neal.  The Dark Side of Nowhere: A Novel.
Smith, Roland.  Zach's Lie.
Spradlin, Michael P.  Spy Goddess series.
Thomas, Jane Resh.  The Counterfeit Princess.
Williams, Mark London.  Danger Boy series.
Zindel, Paul.  The Gadget.