I don't like this one as much as The City of Ember. It answers some questions I had from the first book, and I like that it's set in northern California. But everything is a little too spelled out for me.
I also don't love that this book's epigraph is from MLK's Strength to Love seeing how this is a book about white people.*
DuPrau writes tension well and the pacing is pretty good. But unless there's something truly integral to understanding the fourth book, I'd skip this one if I ever reread this series again.
*No race is ever explicitly mentioned, and the characters can all be read as PoC, but since white is the default in America I'd bet most people read the books assuming all the characters are white (with maybe the exception of the two characters with Hispanic surnames).
ETA: I might go back to see if it tracks, but 28 days seems too long for Lina to have been gone. Doon's storyline seems like it could fill 28 days, but Lina's felt shorter than that to me.