6 books from the X Bar X Boys Series by James Cody Ferris. Books #2, 3, 5, 7, 9, and 13 in the 21 book series.
Boys in Thunder Canyon (1926)
Boys on Whirlpool River (1926)
Boys at the Round-Up (1927)
Boys at Rustlers' Gap (1930)
Boys Lost in the Rockies (1930)
Boys Branding the Wild Herd (1934)
Binding is worn and most and shows its age. These may be first editions judging by the copyright dates. They do not have their jackets.