I think it depends on what kind of book you want to write!

If you want to write a cool science fiction book featuring the Sun I think you just need a good imagination.

If you want to write a non-fiction book about the Sun to educate people, that's always useful! The Sun is a complicated thing, so I think it would help to narrow down the topic a bit. Would you write about how the Sun was created? How it creates its different layers? The solar activity and how it occurs? The influence of the Sun on the rest of the heliosphere? There are lots of different angles to take here.

The general path to writing a non-fiction book would be to learn as much as you can about the Sun (specifically the aspects that your book would focus on), and then figure out who you want the book to be for and craft it to best communicate to that group. Knowing the audience for the book is really important because you wouldn't explain the Sun's activity in the same way to kindergartners and to adults, or to scientists and to non-scientists.

There are a number of books about the Sun out there, but our understanding is always changing and I think more books is always a good thing!