(If you want to read a shorter review of the book by me, I'll link my Goodreads review at the end).
Kathryn Lasky's Daughters of the Sea series is consistently the best mermaid series for young adults. It has a certain feel to it that hasn't been mirrored anywhere else.
The best aspect of this installment in particular is the close attention to detail and research that Lasky has put into all three of the books. Book 1 focused on the servants, two on the a lighthouse keeper's daughter, and three on a preacher's daughter entering high society. All three settings were written so well, showing a lot of diversity on the author's part.
As far as characters, tons of new people are introduced in Lucy, and to be honest it gets quite confusing sometimes. But we also get to go back and visit the previous characters like Hannah and May, as well as the mysterious Aunt who may or may not be the focus of the next book.
If there is one thing to complain about, it's the pacing. I had a hard time initially getting through Hannah because the middle, while enjoyable, stretches out longer than it should. May didn't have that problem, but Lucy does however. It's literally the only think I can think of complaining about for this series.
Unlike a lot of sequels that have been released in young adult this year, stuff actually happens in Lucy. Whereas such promising series have fallen flat in the plot area recently (I'm looking at you Chemical Garden and Matched) Daughters of the Sea keeps going even if the plot points aren't incredibly action filled. Lots of the important bits in this particular book are woven with the complex high class setting.
This series is aimed more towards middle grade than young adult, but it's a sweet reprieve from all the over-the-top romance and faux complicated stories of the older young adult genre books. Definitely recommended.
(Here's my shortened and slightly less in depth review of the book on Goodreads)