As you can see with my halting attempt at Spanish reviewing, I'm fairly familiar with the Harry Potter series. I'm not a die-hard fan--in fact I found the staggering popularity (something like 400 million copies sold) rather bizarre, especially for the later books, which seemed to get worse and worse. Certainly other young fiction is comparable or superior.
In any case, one of the advantages of the that popularity of the series is the wide availability of the book in many languages--67 so far. My current rather feeble attempts at learning Afrikaans center mainly around deciphering dialogue passages in my translated copies. It's not a very effective way to learn a language, but as I don't have any native speakers on hand I have to make do.
Reading books is best practice for people who are already fairly good at a language, I think. If one has a decent vocabulary and grammar, pounding thousands of words helps build the instinctive knowledge that one needs to be truly fluent. For me, the Spanish was at about the right level. I know that if I were serving in a Spanish-speaking country I would be completely fluent by now (sigh). Reading the Afrikaans books doesn't help that much--yet. But I figure language learning is one of those things that rewards time and persistence more than technique. I'll keep after it.