I did not think about this solution to have one more way to increase the power level, but if tomorrow with an update they had to implement the breakthrough skills of a ninja that is part of your main team, in order to increase their effectiveness you would immediately try to upgrade his skills. If once you have increased you find the book as a gift for an event, considering what is worth in terms of fragments, coins, coupons and time is ridiculous that then remains a useless object that you can sell for only 100 coupons, its value becomes really null. With my proposal, if for example first you need 50 fragments of that ninja to get that book, you recycle it and recover half of the fragments (I throw them as an example because to recover all the fragments needed to get the book seems excessive) . In this way, if you have already previously obtained the skill book and find another one at least is not completely useless ... maybe you could put limits on the use of those fragments, similar to the seals scroll free of the events, namely the fragments obtained by recycling the book can only be used to make other skill books for skills not yet enhanced, or to increase the stars of the ninja and not to refine fragments for the jar (in case this is considered too much by the developers)