This is the 755th most frequent Croatian word.
neku
"Some" (feminine singular accusative form).
In this sentence, 'neku' modifies the noun 'knjigu' (book), serving as an indefinite article suggesting an unspecified object.
Kupila je neku knjigu.
She bought some book.
In this context, 'neku' expresses the speaker's intent to find an unspecific interesting story.
Tražim neku zanimljivu priču za čitanje.
I am looking for some interesting story to read.
Here, 'neku' points to an unspecified individual, implying the speaker did not recognize the girl.