Okay, so I have a new least favorite... I suppose it's a good thing I started with the books I did or I would never have given this series this much of a chance.
My biggest issues here are that Morgan's note is written in cursive and then never repeated in print. Even my 3rd graders didn't start learning cursive until September... and from what I understand this series is directed at children even younger than that. It's incomprehensible to me that they'd make a book designed to encourage children to read longer books independently and then put a major plot point in a way that's inaccessaible to them.
There also didn't really seem to be a plot in this one. They arrive, they're carried by the ninjas to meet the ninja master (and, btw, just reading "ninja" and writing it there, I couldn't help but grin sarcastically... :P) who immediately sends them on their own back to the treehouse. And later this "master" tells them that they couldn't find the book because they had a mission to complete. What mission? Pretending to be ninjas is a mission now?
Ugh. This one was just silly. And the illustrations, while still beautiful, just served to remind me I was reading about ninjas, and therefore enhanced the silliness.