This is the 16470th most frequent Russian word.
рятувальників
"Рятувальників" is Ukrainian, not Russian, and translates to "rescuers" in English.
Here, 'рятувальників' is in the genitive plural form, indicating the possession of the dogs belonging to the rescuers.
Суки рятувальників навчені шукати людей під завалами.
The rescuers' dogs are trained to search for people under rubble.
In this context, 'рятувальників' is used in the genitive plural to indicate the topic or subject of the books, which is rescuers.
Книги про рятувальників популярні серед дітей.
Books about rescuers are popular among children.
In this sentence, 'рятувальників' is used in the accusative plural form to denote the people who are rescue workers being called for help.