Lots of people practice their target languages by reading Harry Potter. However, HP is actually pretty advanced, and if you make it through the series, you will be able to read most other fiction.
Rather than slowly slogging through Harry Potter, why not work up to it by starting with easier books? There are other books and series available in your target language that will be easier to read and help you build up you abilities. Then when you do end up reading HP, it will be a lot more enjoyable.
I speak from experience: when I was first learning German, I found a German copy of HP1 and slogged though it. I only had the vaguest idea of what was going on in the story, and I certainly didn’t get into the flow of reading. I eventually abandoned the book a hundred pages in, exhausted. In contrast, with French, I started reading shorter, easier books. Now I’m comfortable enough with reading to tackle Harry Potter, and the process is easier.
Here are some books and series that you should think of reading before Harry Potter:
This series also features teenagers with powers, but the characters fight off a covert alien invasion rather than the Dark Lord. This series is actually pretty advanced, only slightly less difficult than HP. However, the books are shorter than HP, with most Animorphs books taking less than 200 pages. Since there are many books in the series, if you get in the habit you can keep reading.
3. Roald Dahl Books
You probably read some of these books as a kid, or at least saw the movies. There are lots of books to choose from, and they are a good stepping stone after you feel comfortable with the easiest books. The benefit and challenge is that each of these books is self contained, so you have to figure out what is going on from scratch when you start reading each book.
2. Magic Treehouse
When I was learning French, this was one of the easiest series that I could find. In these children’s’ books, a couple of siblings find a magical treehouse, which transports them on adventures in time and space. There are lots of books in the series, so you can keep reading them until you feel like your ready for more difficult material.
1. Magic School Bus
This was the most basic series that I could find while learning French. These books are good because each book in the series is standalone, so you can read them in any order, according to your interests. The books in the French version are novelizations —there aren’t many pictures, and the text flows as in a normal book. In other languages it appears that the series follows the original English version, with lots of pictures and with the narrative balanced against short snippets of text that explain the science.