Based on what I’ve seen at the HTLAL forums, the Harry Potter books are often some of the first books that people will tackle in their target language, as they’re fun books that lots of learners have already read in English. Getting hold of copies in your target language is now a lot easier, provided you’re studying French, German, Italian or Spanish: the Pottermore Shop now has the whole series available for purchase. The eBooks are compatible with lots of devices: e-readers, smart phones, tablets, as well as pretty much any computer.
If you own a Kindle and have an Amazon.com Prime Membership, you get it even better: you can read the books for free. Starting June 19th, the whole series will be available through the Kindle Owners’ Lending Library.