When my partner and I were planning an extended A.T. hike, we stopped by our local outdoor shop and asked the manager what to bring. “Foremost thing you’ll need is this,” he said, reaching for a plain green book which otherwise looked a lot like a high-end car manual: David ‘AWOL’ Miller’s The A.T. Guide.

“This guy, David ‘AWOL’ Miller practically lives on the trail.”

Who was this AWOL? And how did the little green manual come to be?

Back in the late-90s, working as a high-powered software engineer, Miller got turned on to the notion of an A.T. thru-hike via a newly retired, Katahdin-conquering ex-coworker. Miller soon viewed the 2,181-mile adventure as a potentially kickass springboard into his own retirement. But being married, a proud father of three, and not quite 40 years of age, Miller couldn’t conceive of himself as being capable of responsibly hanging up the corporate boots for at least another two decades.

However, despite the pragmatic rationale—must clock the time, take max. advantage of IRA + 401k plans, continue gratingly, monotonously grinding along toward reaping the so-called Golden Years bounty—the A.T. had already infiltrated his system. Miller found himself facing an inconvenient truth: “I remember becoming increasingly aware this thru-hike was something that couldn’t wait. My life had grown precariously normal… It had to be done now.”

In the spring of 2001, having procured a greenlight from the familial quintet, Miller quit his nine-to-five and lit out for Springer Mountain, the A.T.’s southern terminus.

“My wife was actually happy for me to have the break from my job,” chuckled Miller. She and the kids set out to have a memorable summer of their own—beach trips, camping, and visiting relatives. I missed them, but they were aware I was doing something important I had to do for myself, and that was something they wanted to support.” He quickly earned his trail name AWOL after telling his life story to fellow thru-hikers of quitting his job and setting out for the woods. After he completed his thru-hike, he penned a memoir: AWOL On the Trail.

A few years later, in 2007, Miller heard Dan “Wingfoot” Bruce was retiring from the business of maintaining his yearly-updated Thru-Hikers’ Handbook. As a software engineer, Miller realized he could present the trail data in a fresh, new way that was both functional and aesthetic.

“For the next year, I spent every spare moment available working on the book, to the point of enlisting my wife and kids,” he said. His efforts yielded the most intricate, detailed, and stunningly resourceful trail guide yet produced.

The popularity of The A.T. Guide is not surprising. Every year, Miller or a member of his team personally hikes each section and travels to each town to ensure the information is 100 percent accurate and updated.

Miller himself isn’t planning another thru-hike anytime soon. “But if one of my kids wanted to do it and wouldn’t mind having me along, I’d definitely go with them.”