I found The Infinite Sea to be a weaker version of The 5th Wave. The 5th wave was original, had a great atmosphere and was tense throughout. The Infinite Sea is something like an intermission and it answers zero no none questions about this series. 
 (view spoiler)[ The Ringer narrative was refreshing and carried the story on, but in my opinion it was too long and in the been-there-done-that category. Ringer got her Razor who is a traitor turned ally; he helped her recover, nursed her, helped her run, helped her keep her promise (I am pointing to Cassie and Evan arch here). The narrative does serve one purpose though, which redeems this book for me- and that is that 2 books in and there are really no aliens around at all. What the first book identified as an alien turns out be brainwashed humans killing humans. So 5th wave= brainwashed children, the infinite sea= brainwashed humans thinking they are aliens. (hide spoiler)] 
 And it also got me thinking- what is with the mostly female perspectives? I do like the subtle shift in the recent years towards female heroines, mostly turned vigilantes The Hunger Games Divergent or even Twilight to name a few, but an antisocial girl (or two in this case) on a mission meets boy lacks variety regardless of their circumstances. Just being honest here. 
 The Last Star is next.