The Hitchhiker's Guide to the Galaxy

Almost every fictional book in this catalogue is an object other characters talk about. The Guide is the only one that gets to talk back, and that is why it works.

Adams’ real invention was tonal. The reference-book voice — flat, confident, faintly bored, willing to spend four hundred words on a digression and one clause on the destruction of a planet — turns out to be a superb comic instrument, because its confidence never wavers no matter how stupid the content gets. The joke is never in the fact being wrong. The joke is that the Guide’s editorial standards remain visibly intact while it reports it.

Note the commercial detail, which is the most Adams thing in the entry: the Guide outsells the Encyclopaedia Galactica not because it is better but because it is cheaper and less frightening to pick up. A cosmology built on the observation that people buy the reassuring book.

On the readability rating. Extensive. Entire entries are reproduced across the radio series and the novels — Vogon poetry, the Babel fish, the Total Perspective Vortex, the Pan Galactic Gargle Blaster. You could assemble a short book from what has been quoted.