If you don't know what the book is about I'm going to copy and paste some of the short summary that I quickly wrote on Goodreads. Basically you have dual narrators, Sam and Grace. Sam is a werewolf who was bitten when he was ~7-years-old and since then can only switch back to his human self in the summer, as the hot temperature makes him more stable that way. During the rest of the year he's stuck in his wolf form, especially in winter, as the cold temperature makes the animal part of him more prominent. So, temperature regulates the transformation between being a human and wolf. Grace is just a regular 17-year-old who coincidentally was bit when she was very young as well by a pack of werewolves outside her house, but survives the event. She remembers Sam's yellow eyes ever since her near death encounter and for the next six years (up until she's 17) they have this weird romance thing going on, where she is constantly on the lookout and seemingly obsessed with "her" werewolf. Likewise, Sam lingers in the forest behind her house in his werewolf form looking after her all these years until one night when things begin to unfold and he finally approaches her in his human form.
*WARNING: Spoilers ahead, if you have not read the book do not read on!