If you have reads who love series, Haddix is a great choice. The Missing series has seven books, as does The Shadow Children series, which my daughter adored. Ever since Under Their Skin and Children of Exile, reading Haddix's books makes me uneasy. I keep suspecting really weird things to happen. Or, as I put it in one of my reviews, I don't quite believe the worlds that she constructs. They are just too odd. This, of course, is why my daughter liked the books. This is actually something that appeals to children about world building; it just doesn't appeal to me!
Haddix is a local writer, so many of my students have seen her speak, and there is always a demand for her books. I was really intrigues by her notes at the end of the book about Rheta Grimsely Johnson writing "about a sad, odd, true coincidence" that inspired this story, and now I want to know what that was!