Crimson Flame

This book is one of the best paperbacks in the series. It starts off with Frank and Joe breaking up an attempted mugging on a New York subway. It turns out that the mugger is after the victim's newly purchased ruby, called "The Crimson Flame". The victim is a prominent rancher in Arizona. Soon after arriving in Arizona, the victim asks the Hardys to investigate strange events at his ranch. Here they encounter sinister ranchhands and a suspicious staff, and a clue from a hat they find takes them to Thailand and then back to Arizona where they solve the mystery. I don't want to give away too much. Numerous encounters with the criminals happen throughout the book, and the action is non-stop. Their friends, Chet and Biff are in the story, but they play no significant role in the investigation other than be ranch hands at the victim's friend's ranch (and that's not saying much). Highly recommended, great read from start to finish.