El mar infinito

The main character in this book is not Cassie, but Ringer. I didn't like it at first, because she was not a character that I found interesting in the first book, but this had changed by the time I reached the end of The Infinite Sea. I liked the chess analogies and how Ringer's plot can be read like a chess game.
Then, there is the big revelation at the end, which I won't explain, but you'll know what it is if you have read the book. It's not something that I had expected, but it made me look forward to reading the last book. It may disappoint some people, because it changes the meaning of everything, but I hope that it wasn't a red herring to confuse Ringer.
I was expecting to know more about Evan, which did not happen, but if you don't mind that the plot is more psychological than action oriented and you're expecting it, then it's not a bad book.