Answer:
according to her website (stepheniemeyer.com) midnight sun will not be published in the near future if at all. appearently someone leaked an early copy and pissed her off. so shes writen 5 published books 1 somewhat rough draft of half a book and a book that is only shared with her family. the family book is one that was saposed to be after twilight (before New Moon existed) and was going to be the second in the series but was considered too ronchy for YA. so again that's 5 books, 1 draft, and one not shared novel. check out her web site for more info
its actually 6 books twilight, New moon, Eclipse, breaking dawn, midnight sun(not yet published) , and The Host