The book and the movie share a premise and many of the same characters but are quite different. The book is basically plotless, a collection of of vignettes that all share a theme of expectations gone awry, and the title character's heroic struggles simply to endure what's meant to be a pleasurable and relaxing time in a summer vacation town.

You can see how bits of the book crept into very different scenes in the movie—getting lost in the fog in a boat, the bird-watching guests—but the movie version intensifies and amplifies the people and events, always effectively and humorously.

The book is quite funny as well, though it's a quieter, more whimsical, stealth sort of wittiness. James Thurber was very popular at this time and I think it's fair to say that the tone is meant to be very similar to his. Certainly the original hardcover edition's many original drawings, by Dorothea Warren Fox, are meant to remind readers of Thurber's own illustrations for his writings.