"Farmer Giles of Ham" (in the vulgar tongue) is an entertaining little tale, maybe the first real comic fantasy novel (actually, more like novella). It's certainly not on par with his tales of hobbits and elves, but it's still cute, funny and very well-written.
Aegidius de Hammo (or in the "vulgar tongue," as Tolkien archly tells us, Farmer Giles of Ham) is a pleasant, not-too-bright farmer (a bit like Barliman Butterbur) who leads a fairly happy life. Until the day his excitable dog Garm warns him that a giant (deaf and very near-sighted) is stomping through and causing mayhem. Giles takes out his blunderbuss and takes a shot at the giant, and inadverantly drive him off.
Naturally, Giles is hailed as a hero. Even the King is impressed, and sends him the sword Caudimordax (vulgar name: Tailbiter), which belonged to a dragonslaying hero. By chance, the not-so-fierce dragon Chrysophylax Dives has started pillaging, destroying and attacking the nearby areas. Can a not-so-heroic farmer drive off a not-so-frightening dragon?
It's a fast, fun little adventure story with blundering giants, greedy dragons and unlikely heroes (the last one is what Tolkien always does best). It's not epic and it's not deep, but it is entertaining, especially for people who enjoy comic fantasies. You'll like this if you enjoyed the cuter moments of "The Hobbit" and stories like "Roverandum."
Tolkien's writing always seems to be winking at the reader. There are a lot of in-jokes for people who know Latin (the "vulgar tongue" comments) and a lot of cute moments, like the young dragons exclaiming that they always knew "knights were mythical!" And the illustrations resemble old tapestries and paintings, but they usually have a funny sort of twist to them.
"Farmer Giles" is not the deepest or most riveting of Tolkien's works, but it's a straightforward, cute little adventure that kids, adults, fans and non-fans of Tolkien will enjoy.