The Oasis staff won't tell you no early than wednesdays what upcoming ninjas will have skill books as it ruins the surprise. If you want to know which ninjas might get it, there is a google doc on the forum that tells you which ninjas has skillbooks/skillbreaks in China. Hint: The possible ninjas are ninjas you can get when you just started the game so ninjas like lee, gaara, asuma, ino, shikamaru, choji, etc.are what people are expecting for the next batch of ninjas to have skillbooks.

Also, you can choose which ninjas you can refinement as this was an update from 3.0. Just select which ninja you want to refine.