Thought's on an Effective Conclusion

The last novel I read was The Forest of Hands and Teeth by Carrie Ryan.
The conclusion was kind of effective and kind of ineffective. The reason it was ineffective was because it left me with the feeling of “But it’s not finished, what’s going to happen next?” The reason it was effective was because even though I felt that way, I still felt like there wasn’t really anything else the author could have written. It left me with an unfinished feeling that didn’t need finishing!