Blue Lily, Lily Blue (The Raven Cycle #3) by Maggie Stiefvater

ARC
There is danger in dreaming. But there is even more danger in waking up.

Blue Sargent has found things. For the first time in her life, she has friends she can trust, a group to which she can belong. The Raven Boys have taken her in as one of their own. Their problems have become hers, and her problems have become theirs.

The trick with found things though, is how easily they can be lost.

Friends can betray.
Mothers can disappear.
Visions can mislead.
Certainties can unravel.