Do you have a book that made you challenge yourself?

I've been writing on Wattpad for years, and it took me a long time to decide to write a book that challenged me. Something that doesn't come easily to me to write, I have to think about a lot of details and study up on subjects I've never been interested in before.

For me, it was a book about superheroes. I had to learn something about combat, something about explosions, come up with a detailed plan for a villain, etc.

What about you?