While these are wonderful adventures, they are also dealing with the issues that many young people have to deal with as they try to figure out where they stand on the major issues that will effect their lives. Where are the lines you won't cross? What are the important values that you're willing to fight for and maybe lose your life for? They may be young, but that doesn't mean they aren't aware of the world and its problems and their roles in society.
The characters have grown over the series and their actions and reactions reflect that. You can read the books out of order and sequence or as stand alones but they become richer when read in order so that you can see the growth and know what leads to their decisions.