There's a trend in Paranormal YA that I've encountered numerous times over the past couples years. I've named it the New Moon Effect, because that was my first encounter with the trend. This trend involves the dark and mysterious love interest (that just so happens to be a supernatural creature) disappearing in book two. Whether it's because they're kidnapped or they feel it's for the main character's "own good", there are way too many series that use this plot device in the second book. I don't necessarily dislike the New Moon effect when done well, I have just noticed over the years how often it is used. My only problem with it is that when it's used, 99% of the time it's used in the second novel of the series.
Besides New Moon, there are a number of other Paranormal YA series that I've witnessed use this same plot device with varying degrees of success. There are probably plenty more Paranormal YA series that use the New Moon effect in book two that I probably have on my shelves waiting to be read, but here are a few that I have read:
1. Blue Moon by Alyson Noel (have read books #1-2)
I've seen many readers point out numerous similarities between this series and the Twilight saga. I personally thought Evermore was pretty unique when compared to Twilight, but Blue Moon hit a bit closer to New Moon. I almost put this novel down because it's use of the New Moon effect seemed a bit forced and formulaic, but as the story continued to unfold it did get better.
2. Betrayal by Lee Nichols (up to date on this series)
When two Ghostkeepers get together, the stronger of the two steals the powers of the weaker. So the New Moon effect made sense in this novel. Emma couldn't be stealing Bennett's powers if the series was to continue. Bennett's absense was also a much smaller portion of the book compared to other novels.
3. Torment by Lauren Kate (currently reading book #2)
Having only read six chapters so far, I can't speak for the entire novel in regards to this trend. But I knew this title would fall into this category when Daniel talks about leaving Luce along at Shoreline in the prologue, before the reader even gets to check in with Luce.
4. Deadly Little Lies by Laurie Faria Stolarz (have read book #1)
I haven't actually read this title yet, but just from the very end of book one and the blurb on the back cover of book two, I can tell it's going to fall into the New Moon effect. I really enjoyed Deadly Little Secret, so I'm interested to see what book two brings to the table.
On the other hand, there are a number of Pararnormal YA series in which the New Moon effect is absent (I'm not saying these books are automatically better, they're just different). My absolutely favorite example of a book two straying away from this trend is:
Spells by Aprilynne Pike (have read books #1-2)
What I really enjoyed about this novel is that Laurel, the main character, is the one that "disappears" when she goes to train with other faeries. She leaves her parents and her boyfriend in order to learn more of the skills that should come natural to her. And she's only gone for a couple chapters, instead of the majority of the novel.
Then there's the one series that stands out because it sits inbetween:
Melissa Marr's Wicked Lovely series (have read books #1-4)
Ink Exchange, the second book in the series, draws the reader away from Ash and Seth's story and focuses on other characters. But in the third book, Fragile Eternity, Seth disappears for a while. But the roles are reversed in this relationship: Ash is the supernatural creature, Seth is still human. He disappears to try to find a way to ensure that he can continue to be with Ash. I think this series is a great example of using the trend in a novel that isn't the second in a series.
So basically, switch it up. I'm not saying don't have the mysterious, supernatural bad boy disappear. I just want it to be a surprise when he disappears.