The Cabin/ The Celler - Natasha Preston 2 in one book

The Celler : Summer is trapped in a cellar with the man who took her―and three other girls: Rose, Poppy, and Violet. His perfect flowers. His family. But flowers can't survive long cut off from the sun, and time is running out... The Cabin: A weekend partying at a remote cabin is just what Mackenzie needs. She can't wait to let loose with her friends. But a crazy night of fun leaves two of them dead―murdered. With no signs of a forced entry or struggle, suspicion turns to the five survivors. Someone isn't telling the truth. And Mackenzie's first mistake? Assuming the killing is over..