therein lies idiom
那一点就是，此即
in what has just been mentioned we see the reason or explanation for (a particular situation, quality, or problem)
Her book is simply a collection of memories, told without conceit or self-absorption - and therein lies its power. 她的书只是一本回忆集，既没有自高自大也没有顾影自怜——而这正是该书的魅力所在。