There's something about a good YA book that makes it very moving. It's not just nostalgia for the adult readers, because the really good YA books touch on something that transcends that, and speaks to both younger and older readers. I think it has to do with truth, and the way in which we sometimes tell the truth by lying (such is the nature of fiction). In this way, Stargirl is one of those really good YA books.
The premise is pretty simple -- a new student at a high school is quirky enough to attract enough attention to become popular, but that quickly sours as the other students misunderstand her intentions -- but the storytelling isn't. In fact, the relationship between Stargirl and Leo, the narrator, is complex enough to take anyone back to the days of high school, when a simple, honest relationship can take weird turns that make it into something unnecessarily complex. That's exactly what Stargirl is, a bittersweet story love and loss.
What makes the story so good to read, though, is Spinelli's writing style, which is pared down and succinct. He presents the characters and the idea of the story plainly, but doesn't spare any emotion during his storytelling. In fact, it's easy not to realize that you're reading a book that's shorter than 200 pages, because the book has a powerful emotional punch to it. He even works the setting -- New Mexico -- into the story, almost as a character itself. In fact, I think Spinelli's characters are where he really shines, because you'll find yourself adoring Stargirl, sympathizing with Leo, and strongly disliking everything that conspires to work against them.
This book has received a lot of awards, and rightly so. I'd recommend the book to anyone, young or old.