Day 21 by Kass Morgan

In the heart-stopping sequel to Kass Morgan's 'The 100', it has only been 21 days since the hundred first fell from the sky and stepped foot on Earth. After discovering they weren't the only inhabitants of the planet as originally thought, Wells attempts to keep everyone together. After all, that is the only way they'll survive. Clarke is in search of other colonists while Bellamy's only concern is keeping his sister safe. As the struggle to survive intensifies, everyone's beliefs and relationships will be tested. Can the hundred make it through all that they face? (Book 2 of 'The 100 Trilogy')