Miller, Kirsten. The Eternal Ones.
You've probably seen this in a display of Twilight read-alikes, but this is a huge disservice to the book! This book reminded me of one of the few adults books I've bought, Ferney by James Long, and while it is more of a high school book, I enjoyed it immensely.
Haven has suffered from seizures for years, and as a child had very vivid memories of a past life lived in New York City with a man named Ethan. This has caused her many problems in her small Southern town, and her grandmother thinks she may be possessed by demonic forces. Haven wants to make it through her narrow-minded high school with her gay best friend Beau (with whom she custom designs prom dresses) and head off to New York, but after suffering a seizure after seeing wealthy celebrity Iain, she acts on this earlier than planned. It turns out that she and Iain both have memories of past lives, when they were together, but Haven starts to wonder if Iain is behind both her death in a past life and deaths that are occurring in the present day. Working with the Ouroboros Society, she is connected with others who also have been reincarnated, and finds that there are forces of evil at work.
Again, this was a great book, but Haven's relationship with Iain, even if they were married in a past life, makes this more appropriate to grades 9-12. The beginning is also more philosophically introspective, and middle school students may want something with a little more action.