WHY DO HARRY POTTER BOOKS BREAK RECORDS IN SALES?

I would add that the topics presented in the books are more than just magic and adventure, but also themes that last through generations, such as friendship, honor, integrity, love, hate, fear, bravery, and sacrifice. These themes, no matter what the setting and characters, can give an ordinary story a life of it's own. When a person can identify the characters of a story as realistic representations of true people, then they become like friends and the reader becomes naturally concerned for their lives and futures.

Just as with books like The Lord of the Rings, these themes, when uses correctly and efficiently, can cause a story to become a classic and live for centuries and beyond.

Well stated...

Harry Potter magic
Plus, the characters change and grow up as the series progresses. This is unlike the usual school story series where the characters stay the same age, with the same behaviors, regardless of how long the series continues.

Kids make up a very large market which is somewhat untapped. And most importantly, they are prone to fads (pokemon, cabbage patch dolls, elmo's, etc).

Harry Potter books have a simple (recognizable) name, a good story, and a little bit of controversy to keep everyone talking. A perfect marketing opportunity