So far there are two books in the series.
1) Gone
2) Hunger
There are supposed to be 6 books in the series. One is supposed to come out every summer for the next four years. Grant announced on Twitter that the finished third book will be entitled Lies and will not be released for another year. He has also stated that he is starting to write the fourth book entitled Plague.
The title of the next book in The Dresden Files series is reported to be called Skin Game.
Poison Pen is the next book in Mary Kittredge's Charlotte Kent series.
The next (6th) book in the series, entitled Risked, will be released on September 3, 2013.
The next book in the Twilight series is Breaking Dawn.
Air keep is the title of the next book. It should come out this year, since the author wants to keep the series going. there is no firm date yet. Stay tuned!
The next book in the series is called The Shadow Rising.
I believe you're asking about the last book that's being released in May/2009. It'll be called The Last Olympian.
The next book in the series is Hidden.
The next book in the warriors series will be Sunrise.
Eclipse is the next book of the most famous series Twilight.
The next book is in the "New Prophecy" series, and it is entitled "Midnight."
The book after "Darkness Falls" in the Survivors series is "The Broken Path."