Ask HN: How much experience is required to write a book?

Recently I was asked to review an outline for a book on a relatively new programming language which I have been tinkering with since it was released. After reviewing the outline for the publisher I was asked whether I would be interested in co-authoring this book.

I feel kind of conflicted about this, because I always try to be fairly modest about my abilities. I am not a member of the core dev team for this language, and have not contributed to the standard library. However, I feel like at some point you have to give yourself some credit and say "yes, I do have the expertise to advise other people on this topic".

I know we have quite a few published authors floating around on HN who will have experience in this situation, and i'm sure would have faced something similar when writing their first book. When did you feel comfortable putting your experience on a page? When did you feel confident that your experience would be valuable to other people?

At this stage i'm looking to turn down the offer because I don't think I can honestly say that I am an authority on the topic for the proposed book. The way I see it, someone who is writing a book should be an active member of the community at the very least, but i'm more of a lurker. Aside from being active they should also have some recognition within the community, which generally comes with activity and contribution to such a community. For me, the other edge of this sword is that people often become authorities by publishing a well-written pieces of literature that the community feels are valuable.

Is there some critical balance between having valuable experience/knowledge, and simply believing in yourself enough to take up an opportunity like this?