The title’s “uncommon reader” (Queen Elizabeth II) becomes obsessed with books after a chance encounter with a mobile library. The story (follows the consequences of this obsession for the Queen, her household and advisers, and her constitutional position.
The title is a play on the phrase “common reader”. This can mean a person who reads for pleasure, as opposed to a critic or scholar. It can also mean a set text, a book that everyone in a group (for example, all students entering a university) are expected to read, so that they can have something in common. A Common Reader is used by Virginia Woolf as the title work of her 1925 essay collection. Plus a triple play – Virginia Woolf’s title came from Dr. Johnson: “I rejoice to concur with the common reader; for by the common sense of readers, uncorrupted by literary prejudices, after all the refinements of subtilty and the dogmatism of learning, must be generally decided all claims to poetical honours.”
In British English, “common” holds levels of connotation. A commoner is anyone other than royalty or nobility. Common can also mean vulgar, as common taste; mean, as common thief; or ordinary, as common folk.