The Indian Horse Mystery, by Mary Adrian (epub/Kindle/pdf)
"Harold "Hap" Miller, whose parents owned the Hanging Rock Ranch, is best friends with Jim Hawn, a boy of the Yakima Indian tribe. When a calf goes missing on the ranch, the two boys ride out to find it -- and set off a chain of events that reveals a modern-day mystery!
"Another fast moving Mary Adrian mystery. Like the others, into its plot is woven a lot of interesting information about life on a western ranch and an Indian reservation." -- The Times Recorder