This book could be a type specimen in the argument Daniel Abraham was exploring just yesterday about the dynamic tension between sentence and story. One could write a perfectly decent bit of post-apocalyptic science fiction about the recovery of lost knowledge and the dynamics of social and technological power using the plot, characters and setting of Riddley Walker. That's not what Hoban did. He wrote a puzzle story, where the puzzle is in the framing, phrasing and vocabulary of the story — a technique for example much deployed by Gene Wolfe among others, but Hoban takes it to a grand scale. That layer of linguistic manipulation completely shifts the book away from the underlying story it tells and pushes it into another sphere entirely.
Riddley Walker is written in a mode very reminiscent of eye dialect. In point of fact, this is not eye dialect, in the sense that the narrator is explicitly writing things down rather than having his speech quoted. He lives in a world that barely has orthography, let alone dictionaries. Spelling is remarkably eccentric, yet largely phonetic. The sense of the culture that comes through Riddley's word choices, and the reader's efforts at comprehending how the meanings have shifted, is a huge part of the experience of the book.
Science fiction writers especially use linguistic evolution as a story telling and worldbuilding tool, but most of us don't do it with every damned word on the page. This is impressive, and challenging. To say the least.
One thing I struggled with was sorting out which of the linguistic transformations were literary devices of the book, and which were my own misunderstandings of the substrate of English culture embedded in the story. For example, it took me quite a while to recognize that "Pry Mincer" was a corruption of "Prime Minister", and I didn't grok the connection between the Eusa shows and Punch-and-Judy until that was explicitly stated fairly well along in the book. This is not a complaint, just an observation. And in fact, it's that selfsame lexical detective work that makes the book both so much fun and so much work. Sorting out the subtle expansions of meaning in the words "hevvy" ("heavy") and "foller" ("follow"), for example, occupied me considerably.
My best advice, swiped from tillyjane (a/k/a my mom) is to read it aloud. Much like Huckleberry Finn, this book makes more sense that way. Though I never did figure out what "sarvering" meant in the phrase "sarvering gallack seas and flaming nebyul eye". (The Internet has since informed me that "sarvering" means "sovereign", but I'm not sure that makes much sense, either.)
At any rate, you will likely either love or hate this book. I don't see much room for ambiguity. Enjoy.