(2) Sea Level
Nation isn't just at sea level—it's at sea level on a beautiful tropical beach. One that, er, wasn't wiped out by a massive tidal wave. Sorry, Mau. Yes, the book deals with heavy issues, but it does it with style and humor that make it fun to tackle them. The two main characters are in their teens and their vocabulary is right in-line with this age group, as if in a hyper-verbal Dawson's Creek. Take this, from the very first paragraph:
It was four in the morning and the place was coming alive and Captain Samson had never seen the dock in such a bustle. (1.1)
Yeah, you got that. Pratchett occasionally uses some lovely language and imagery, but it never gets much harder than this opening. Also, one of the characters doesn't even speak English at first, so sometimes they communicate with stick figures. Everyone loves stick figures! It doesn't get much easier than that. (Just remember: "easy" doesn't mean "dumb.")