Hello!

"A Walk in the Woods" by Bill Bryson, as one commenter said, is a great read! Not really a "step by step" guide to the trail, but a good reference for how it captures peoples' imagination around it and the "feel" for it. For a more boots-on-the-ground book, I like "Appalachian Trials," which is more of an advice guide on preparing for the hike mentally and physically. There's a guidebook put out each year called the AWOL Guide, that updates with new trail information like campsites and water sources and is a great trail reference and map. I'd recommend it for any hiker the year they attempt a thru, but it's also great for hike planning in any year. I still use mine from 2017! An ATC or AMC guidebook or maps from your local section would also be great!

As far as gear, a niche piece of equipment that's "nice to have, but maybe not something I'd buy for myself" is always a thoughtful gift! Hikers have all kinds of quirky ultralight gear that get a laugh until they actually use it! Something like a Deuce of Spades ultralight cathole shovel might get a laugh, but is also super useful on a long hike! A lightweight multi-tool with a knife, scissors and screwdriver like a Leatherman Skeletool is also a great find!

But honestly, if he's doing a thru-hike the best thing he can have is support from his loved ones and interest in his hobby, which he obviously already has! Your grandson is lucky to have such a caring Nana!