Don Winslow, a former private detective, has written “The Power of the Dog” trilogy. It deals with the Mexican drug trade. (He recently challenged President Donald Trump to a border debate.) The latest in his trilogy, “The Border,” went on sale Tuesday. It is a fascinating, well-researched epic of the drug war in the 21st century. Featuring conflicted Drug Enforcement Administration agent Art Keller, this gritty series is a must read for anyone who seeks to understand the war on drugs and what has …