I just reread books one through six, and I’m on the library waiting list for seven.
Book one was childish and simplistic, but they have ramped up from there. They are by no means deep literature, but for an adult, they are pretty good light reading (and books five and six may be up there as light-ish rather than light).
The settings and props are wonderful and inventive, and the main characters get pretty well developed over the course of the series. The kids start as straightforward tweens in the first book, and move into their awkward and somewhat troubled teenage years as they go.
The first one pretty much was confined to Hogwarts, but the others bring in elements of the broader wizarding world, including its politcs and international affairs, as well as characters who straddle the line between good and evil.
I don’t know which thousand unread SF and fantasy books you have on your shelf, but I doubt if many of them are even a fraction as important and influential the Harry Potter series is and will be. The fact that tens of millions of copies of book seven are being read this week around the world (not to mention the tens of millions who have seen movie five in the past few weeks) shows the power of the series.
The series is and will be the seminal fantasy experience for at least a generation of kids growing up. For each one of us in our mid-twenties or older who were drawn into speculative fiction by Tolkien, Narnia, The Phantom Tollbooth or any of the other classics, there will be two in the next generation of readers who cut their teeth on Harry Potter. Not knowing the vocabularly of Harry Potter, will be like not knowing who Bilbo, Frodo or Gollum are.
It’s not that that they have the majesty or heft of Lord of the Rings. Indeed, it’s almost the opposite in that are highly accessible to readers who just want a good story and don’t worry too much about making sure all elements of the world around the characters are fully fleshed out (which they aren’t).
All of that portentiousness aside, the books should be fun summer reading for you, and I’m sure you’ll have a better time rolling through them than 90% of the stuff on your shelf.