Some people speculate that C.S. Lewis's description of Eustace's parents in Chapter 1 parodies Mormons or Seventh Day Adventists. However, Lewis's adopted son, Douglas Gresham, said that the resemblance is accidental and that Harold and Alberta Scrubb are meant to be "faddists" – people who are into all the unusual lifestyle trends of their day. We imagine that if they lived in 21st-century America, they'd be avoiding wheat gluten, wearing those shoes with the round bottoms, and convincing Eustace to try speed dating when he grows up (source).
C.S. Lewis slightly revised The Voyage of the Dawn Treader when it was first published in America. In 1994 the publisher decided to revert back to the "original" British edition as the standard text. As a result, there are two versions – the pre-1994 American edition and the contemporary American edition (which is the same as the British edition).
The changes, which are minor but interesting, are mostly in Chapter 12 and the adventure at the Dark Island. Here's a quick test to find out which edition you have: Look at the end of Chapter 12. In the British/contemporary American edition, Lord Rhoop asks Caspian to promise "Never to bring me back there." In the pre-1994 American edition, Lord Rhoop asks Caspian to promise "Never to ask me, nor to let any other ask me, what I have seen during my years on the Dark Island."
If you want to know more about the two versions of the book, we suggest you read Chapter 2 of Peter J. Schakel's Imagination and the Arts in C.S. Lewis, part of which is available through Google Books.