In the Harry Potter series, J.K. Rowling keeps her world balanced. She doesn’t introduce new magical concepts at key points in the story. She doesn’t give create characters with important roles without previously developing them. There are plenty of plot twists and unexpected things, but everything that happens magically has a previous appearance and is fully incorporated into the story before it’s needed. I have read many good books, but only Harry Potter has this amount planning put into it.
Spoiler Warning! (and if you still haven’t read the books, then what are you doing reading this post? Go and read them now!)
One example of this preparation involves Portkeys. In the beginning of The Goblet of Fire, Harry takes a Portkey to the Quidditch World Cup. At the time, this seems to simply be a form of wizarding transportation to bring lots of people to the same place, but the Portkey plays a much more important role in the story when it brings Harry to the graveyard. Had J.K. Rowling not explained this at the beginning, then we would have wondered why all of a sudden touching objects could take you places.
At times she developed characters that didn’t even end up having a role in the final draft. Florean Fortescue, the owner of an ice cream parlor in Diagon Alley, was initially planned to be the conduit for the clues that Harry needed to receive during his quest for the Hallows and Horcruxes. Rowling introduced him early in the third book as a kind shop owner who helped Harry with his History of Magic homework. Four books before he was even to have an important role, she was setting up and building his character. She even had him kidnapped by Voldemort in book six, but he never made the final cut and ended up using Phineas Nigellus Black and the Grey Lady, but the preparation and dedication still shows.
The world is well developed, there are many references to things that don’t have to directly do with the story such as sub-departments in the Ministry of Magic, names of classmates to pull upon when needed, color themes, and illegal flying carpet trade. All of these extra details add depth and context that enhance the quality of the book and the enjoyment that comes from reading it.