Ask HN: Do I need to be an expert on some technology to write a book on it?

I recently published a book. It's about Django, a technology I've been using for more than 5 years now. I consider myself pretty good at working with Django which is why when a publisher approached me to write it I was happy to jump at the opportunity.

I agreed to write the book because I though it would be a nice challenge, and I couldn't miss the chance to call my self an author on my profile! But having gone through the year long process of writing and publishing it, I now would love to do it again. The whole process felt pretty awesome, and I learned a ton of new things about Django that I would not otherwise have looked into. I have approached a publisher with an idea for another Django book but they say that the target market isn't big enough. However they say they'd be happy to hear about other ideas from me.

What I'd like to ask the HN crowd is that should I be experienced in a technology before I can even consider writing about it? Or should I just pick something I'm interested in and start writing, figuring out the details along the way? Do you guys have any experience with this?