I bought this book a few months ago and was just now able to find time to read it. I have to say, I liked it much more than the first book. The main character, Jedediah, was much more likeable than his elder sister was in the first book. He also seemed to be better developed than his elder sister had been. From the beginning of the book to the end, Jedediah grew from the mindset of a boy to that of a man, but was still able to maintain his playful nature and not become dull or boring; It was really fun to see that growth happen in him. There were a lot less grammatical errors in this book than there were in the first one; it made for a much more enjoyable read. I was happy to see that the characters talked much more modernly in The New Guardian, as opposed to the non-stop, old-timey, proper talk that was in The Legend of The Mer. It was a slow, relaxed read that didn't have much action, but it was cute.
I noticed on Mrs. Swift's blog that she plans to write two more mer books that will follow the last two Prentis children. It would be nice to see Mrs. Swift "show" more in those stories instead of "telling" so much. "Showing" will help the stories read more fluently, whereas "telling" causes the stories to read a bit like an outline. Some more action would be nice as well ;). I do enjoy these mer stories though, and am looking forward to the next book. These books are cute and clean, which is very nice. As I said in my review of her last book, these books would be great for pre-teens.
I give The New Guardian a 3.5 out of 5 stars simply because it didn't have as much action and conflict as I was hoping for.