The first book tells the story of Lena and Hana in the months before their procedures. Without getting into a lot of spoilers, Lena falls victim to amor deliria nervosa by falling in love with Alex. In the second book, Lena becomes part of the rebel movement and grows close to Julian, but just as a result of circumstance. In the third book, the rebel movement gains power, and Lena is in a love triangle of sorts with Alex and Julian.
The first book is quite slow with lots of what, to me, is filler. Teenage girls might have a different view. The second book is much more exciting than the first book, and the third book is similar to the second book.
Most any epic story will have at least a partially open-ended conclusion, since the authors typically do not expound in great detail every event that ever happens to the characters. But this trilogy does not successfully resolve the conflict. For me, I just can't see how it could possibly work out. In fact, I could see everyone getting killed one chapter after the book ends. I suppose they could live happily ever after. We really don't know. The author is supposed to give us some idea of how events will unfold. That's not my job as the reader. I am not into writing fan fiction. I am just a reader.
Most readers have the same opinion. Here is a screen cap from Amazon.
I read this book shortly after it was released and found the reviews to be rather interesting as they came in. A good many people complained about the ending and gave specific reasons why. It was apparent they had read the book. Let's just say that after a few too many complaints, a number of five-star reviews would appear, and those reviews were along the lines of "OMG. This was the best book EVER!!!" with no specific information given. I have heard of fake reviews and paid reviews on Amazon, and this book appears to have a number of them.
The Delirium trilogy is excellent, but just be aware that the author apparently wants us to imagine the ending.