It's a little bit odd, I think that the author chose to introduce a new character into the series and give him his own book, but ShaoShu is an endearing little guy, and I suspect he has a much greater role to play in this epic (we shall see, I suppose--one more book to go).
I think that Mouse is probably the most complex book in The Five Ancestor's series (up to this point, anyway). Mouse focuses less on emotions that would be easy for older elementary school kids to understand (revenge, anger, jealousy), and we dive into a world of complex political machinations and power plays. While Mouse features the youngest character in the series, the book is likely to be best understood by middle school readers, as opposed to elementary school kids who would have had little difficulty in understanding Tiger or Monkey.
There is less martial arts action in this one, too, but that's not to say less violence. The violence has taken on a different tone--now it's war, and there are naval battles and gunfights. Gunpowder takes center stage in Mouse while flying side kicks are kept to a noticeable minimum. I definitely prefer martial arts action to cannon fire, but this book also felt more mature to me, so I guess things balance out.