"Secret Vampire" tells the story of James and Poppy, best friends since childhood and partners in crime. Of course, Poppy doesn't know that James is a vampire until she finds out she's dying of terminal cancer and he reveals the only way of saving her is to turn her. Once done, Poppy is thrown into the realm of the Nightworld and understands that James has just sacrificed everything to save her. And now the both of them are in danger of being hunted and killed by Nighworld authorities. 
 "Daughters of Darkness" is probably my favorite out of the entire series--read and reread more times than I can count. It's about three vampire sisters (The Redferns--Rowan, Kestrel and Jade) who flee the Nightworld to live among humans. Once they reach Briar Creek, Oregon, they discover their aunt has been mysteriously staked by someone who obviously knew she was a vampire. They befriend Mary-Lynnette and her brother Mark, two humans who discover their identities quite by accident. Since Mary-Lynnette and Mark are humans, they are now in danger of being killed by the Nightworld. Of course, the Redfern sisters' brother Ash is hunting down Rowan, Kestrel and Jade with the single minded plan of getting them to return home where "everyone misses them violently". The sisters know that if he discovers Mark and Mary-Lynnette know the truth, he'll kill them on the spot. Ash is notoriously malicious, and as stated by Jade, "He HATES humans." Imagine Mary-Lynnette's surprise (and Ash's too, for that matter) when she discovers she and Ash are soulmates, and despite their differences they can't seem to stay away from each other. 
 "Spellbinder" is the third book in the edition. Thea and Blaise Harman are sisters and witches. Blaise is a "bad" witch, and--well--imagine a teenager. And then imagine a teenager with enough power to control the people around her with little moral interruption and you have Blaise. She destroys the lives of human men by getting them to fall in love with her through various spells and then tossing them away when she tires of them. Her sister Thea could probably be described as a "not so bad" witch, holding contempt for humans but has a naturally gentle disposition which compels her to help them nonetheless. When helping heal a human from a snake bite, Thea realizes that Eric is her soulmate and tries her damndest to stay away from him. That is, until Blaise becomes determined to control Eric. Thea knows what happens to the men Blaise controls and can't help herself when she decides to throw caution to the wind and fight her own sister to protect her soulmate.