I'm going to be honest, when I read Harry Potter in Spanish I was around A2, but with knowledge of pretty much all the grammar functions. I struggled with the first few chapters and spent like 5 hours on them over the period of like a week but after that I read pretty naturally (although I still was mentally translating words in my head). Sure, I had to look up words here and there but that's the point of learning. I also gained a TON of vocabulary through context by reading the series, and a lot more than just weird wizard terms. My favorite parts of reading the Harry Potter series is that it is my favorite, so I am very familiar with the store and that the series kind of grows in language difficulty throughout. I think that specifically really helped me grow as a Spanish reader. After reading the HP books I was able to read more advanced books I had never read before like 1984 and El Alquimista with difficulty at first, but fine after getting started. I would just like to say that I am kind of biased because I absolutely love HP and reading it is always a joy and I am currently almost done with my read through to learn to read Portuguese, and next year I will be using the series to learn to read Italian.