Humor is very hard to write. Jerome K. Jerome does a fine job in his "Three Men in a Boat" -- it's a light, frothy sort of humor that brings to mind a 60's comedy movie. The characters are irritable and inept, and the things that tick them off (packing luggage, setting up a tent, dealing with women in nature, cooperative rowing) are still very relevant today. For example, people who hate jet-skis can find parallel rants on the steam-powered launches that annoy Jerome and his sailing buddies.
The occasional dips into seriousness (English history, a single-mother suicide) take up a net 2 pages total, and don't happen to detract from the overall humor. My favorite line is spoken by Jerome to the overzealous cemetary watchman, who can't believe Jerome doesn't want to view some open tombs: "Leave me immediately or I shall climb over the fence and slay you."
The book gets 4 stars only because (like most observational humor) it leaves the mind easily, and doesn't display the unity of meaning, humor and characterization that modern readers expect from a 5 star book. This book is an excellent series of hilarious essays loosely strung together in the form of flashbacks, but it is not a novel per se. One of its greatest achievements is that it is still readable today, quite accessible to a modern audience.