The Uglies Series by Scott Westerfield
Book #1 The Uglies
Book #2 The Pretties
Book #3 The Specials
Book #4 The Extras
The Last Survivors Series by Susan Beth Pfeffer
Book #1 Life As We Knew It
Book #2 The Dead and The Gone
Book #3 This World We Live In
Midnighters Trilogy by Scott Westerfield
Book #1 The Secret Hour
Book #2 Touching Darkness
Book #3 Blue Noon
Green Heart by Alice Hoffman
This is actually 2 books in one:Book #1 Green Angel
Book #2 Green Witch
Divergent Series by Veronica Roth
Book #1 Divergent
Book #2 Insurgent
Here are several others that are great young adult reads:
Matched Trilogy by Allie Condie
Maze Runner Trilogy by James Dashner
Graceling Series by Kristin Cashore
Across the Universe by Beth Revis
Cinder by Marissa Meyer
The Giver Trilogy by Lois Lowry