By J. R. R. Tolkien
A homebody hobbit gets pulled out his own front door and into a dragon-sized adventure, and doesn't come home the same.
Bilbo Baggins is happy exactly where he is: his armchair, his stocked pantry, and no adventures, thank you. Then the wizard Gandalf shows up with thirteen dwarves, and before Bilbo quite knows what's happening they've hired him as their 'burglar' and marched him off toward the Lonely Mountain. The mountain used to belong to the dwarves. Now the dragon Smaug lives inside it, asleep on a heap of their stolen gold. The road there is full of trouble — trolls who want them for dinner, goblins in the tunnels under the mountains, giant spiders in a black forest, and wood-elves who lock them up. Somewhere in the dark, alone, Bilbo meets a slimy creature named Gollum and plays a riddle game with his own life as the prize, and that's where he finds a plain gold ring that makes whoever wears it invisible. There's a town built out on a lake, and at the end of all of it, the dragon. A warm, chatty narrator tells the whole thing and keeps it from ever getting too grim, even when the danger is real.
This is the longest, densest book on the trail, and that's the point of it. Full-length fantasy with unhurried, old-fashioned prose, songs dropped into the middle of chapters, a fold-out map, and a whole world Tolkien clearly thought through down to the last detail. A child who's worked up through the earlier books can handle the length here, and the reward is a plot that never sits still — there's always a next danger, a next place, a next creature around the corner. Bilbo himself is the best part. He starts a fussy little homebody and turns into someone genuinely brave, and you feel every step of it.