This is the 3221st most frequent Russian word.
никому
"To no one" or "nobody" (dative case).
The word "никому" means "to nobody" and is used in the dative case to imply that the action (not telling) will not be directed towards anyone.
Я не скажу никому этот секрет.
I will not tell anyone this secret.
Here, "никому" means "to nobody" and expresses that the book lacks interest for everyone, framed in a negative context.
Эта книга никому не интересна.
This book is not interesting to anyone.
In this sentence, "никому" is used dative case to express that the action (not harming) should not be directed towards anyone.