LOL, HP was the first book I ever read in Hebrew. 🙂 It’s very hard, but most of the hard words are English, and the grammar is complicated. I don’t think it’s a good first book.
If it was cleaner, I would say “Diary of a Wimpy Kid”. It’s geared to children and very entertaining, but you’ll learn a lot of not so clean modern Hebrew along the way.
The best Hebrew book I ever read was Rabbi Lau’s autobiography, ?? ???? ??? ?? ????. Now that it’s available in English it may be harder to resist cheating, but it’s easy and compelling reading.
If you’re even considering reading Hebrew books, you will do fine in Israel.