This is the 13809th most frequent Russian word.
носительницей
A female bearer or carrier.
In this sentence, the word 'носительницей' is used in the instrumental case to indicate that the subject ('она') has the ability to fluently speak Russian, being a native speaker.
Она является носительницей русского языка.
She is a native speaker of the Russian language.
Here, 'носительницей' describes the book metaphorically as a 'bearer' or 'holder' of historical information or knowledge, personifying the object.
Эта книга является носительницей древних знаний.
This book is a carrier of ancient knowledge.
The word 'носительницей' refers in a biological context to the female as the bearer or carrier of a specific genetic trait or gene.