My Soul to Keep by Rachel Vincent {Spoiler Alert}

My Soul to Keep:
Kaylee has one addiction: her very hot, very popular boyfriend, Nash. A banshee like Kaylee, Nash understands her like no one else. Nothing can come between them.

Until something does.

Demon Breath. No, not the toothpaste-challenged kind. The Netherworld kind. The kind that really can kill you. Somehow the super-addictive substance has made it's way to the human world. But how? Kaylee and Nash have to cut off the source and protect their friends--one of whom is already hooked.

And so is someone else...

My Soul to Keep is the fourth book (including the short online novella My Soul to Lose prequel) in the Soul Screamers series by Rachel Vincent. This books begins where My Soul to Save left off, Kaylee and Nash have returned from the Netherworld after saving the soul of Addison Page's little sister, Regan. Unfortunately they were not able to save Addison from dying without her soul and it is now in the possession of the hellion, Avari. Four weeks have passed since their return and Nash has begun lying to Kaylee and is hiding a dangerous secret. In her own unique way, Rachel has breached the subject of teen addictions when it becomes apparent that some of Nash's fellow teammates are inhaling Demon Breath known to them only as "FROST". Meanwhile, in exchange for his services as a courier for Avari between the realms, Tod is given one hour a day to spend with Addison's soul. What Tod doesn't know, is that inside the packages he carries back and forth is the Demon Breath Kaylee and Nash are trying to stop their friends from inhaling. Tod and Kaylee soon discover that Nash has also become addicted to Demon Breath and in order to pay Avari for his supply, Nash has given up everything he ever felt in his memories - those of the past and his brother, and the newer ones of Kaylee. Devastated when she learns that Avari has also been possessing her body in order to speak to Nash. A violation near impossible to forgive Nash, for he allowed it to happen on many occasions, and because during those times Kaylee was completely unaware of what was happening to their body, and what Avari might have done with it. After one of Nash's teammates dies from an overdose of "FROST", Nash and Kaylee's father are kidnapped by Avari and taken to the Netherworld where he plans on using them as proxies (a power source) to open a gateway into the human world, and trapping half the population of Kaylee's school in the Netherworld. Kaylee is contacted by the mysterious Alec, who has been a proxy to Avari for decades and wishes to be free, but in order that to happen, Kaylee must risk crossing over to bring him back with her to the human world. Alec promises to help with the rescue of her father and Nash if she gives her word that she will take him from the Netherworld. Kaylee agrees and with the help of Tod and Addison, they are able to thwart Avari's plans and save her father and boyfriend. Now Kaylee must decide if she can ever truly trust Nash again and whether she can believe that he has stopped taking Demon Breath and that he has beaten his addiction for good.