At least per my copy of The Ultimate Hitchhikers' Guide Complete And Unabridged (a hardcover with all five books plus the short story Young Zaphod Plays It Safe):

The first book, The Hitchhikers' Guide to the Galaxy, starts out with the passage that begins "Far out in the uncharted backwaters of the unfashionable end of the Western spiral arm of the Galaxy lies a small unregarded yellow sun." Later in this passage, you find: "Many were increasingly of the opinion that they’d all made a big mistake in coming down from the trees in the first place. And some said that even the trees had been a bad move, and that no one should ever have left the oceans."

The Restaurant at the End of the Universe starts with a preface: "There is a theory which states that if ever anyone discovers exactly what the universe is for and why it is here, it will instantly disappear and be replaced by something even more bizarre and inexplicable. There is another which states that this has already happened." The beginning of Chapter 1 reads "The story so far: In the beginning the Universe was created. This has made a lot of people very angry and been widely regarded as a bad move."

I might be one of the very few people under the age of 50 to know THHGttG as a radio play first and a series of books second; All of the above and more in the books comes straight from the radio play, but their places shuffled around.