Spoilers for Everneath below. It's really impossible to talk about this book without going into what happened.
The main plotline of Everneath revolved around Nikki trying to find a way to escape the tunnels which feed on humans for energy. Cole wanted her to become an Everliving and challenge the Queen, but she wanted to see her ex-boyfriend and she had six months before the tunnels came for her. She and Jack do find out how to save her and why she survived as a forfeit. Jack dreamed of her each night that she was gone and that kept her memory alive as he was her anchor. To escape the tunnels, someone must take her place.
In the end of the book as the tunnels come for her, Jack pushes her out of the way and takes her place. She resolves to get him back and become his anchor in the meantime, dreaming of him every night. When the next book starts, she still hasn't been able to figure out how to save him and his memories have been getting weaker in her dreams.
Finally Cole comes back to town and she manages to convince him to take her down to the tunnels and rescue Jack. She keeps her memories of Jack alive through remembering him. He doesn't appear in the book, but we get to see important times in their relationship before she left last year.
Overall, I liked the book and I do love Cole. There were some good moments and I've resolved to finish the damn series.
The ending twist, that by kissing Cole (aka feeding in the Underworld which she was warned not to do) three times, she has completed her first step to becoming an Everliving. One thing I didn't really understand was how Jack got back. She does find him, but then is kicked out and he somehow makes it back to her. It made no sense and is one reason for the lower rating.