This is the 10669th most frequent Russian word.
волжских
"Of the Volga" (related to the Volga River).
'Волжских' is used here as an adjective to describe the banks ('берегах') of the Volga river.
Волжских берегах текла река.
A river flowed along the Volga's banks.
'Волжских' functions as an adjective modifying 'достопримечательностях' (sights), relating them to the Volga region.
Я прочитал о волжских достопримечательностях в книге.
I read about the sights of the Volga in the book.
'Волжских' serves as an attributive adjective to 'народов' (peoples), denoting those associated with the Volga.