The kids have started Harry Potter and the Goblet of Fire, book number 4, after many requests. We'd hesitated because the book is darker than the others, with the appearance of Voldemort and the death of Cedric Diggory, among other things. But the kids had learned both those facts this week (thanks to Bud's HP-themed camp), so we relented. It'll still require much discussion, especially towards the end of the Tri-Wizard Tournament.
But they are so interested, we couldn't say no.
Only, I think these tomes, like HP 3, should count as more than one book for the summer reading homework!!