I think that writing a book today is like having a business cards. You let you future customer or employer know that you have a deep knowledge in a field or are able to write well.

Maybe a bit more than a card. Maybe similar to showing your portfolio of past projects. It's impressive but it won't make you rich.

I remember going to a book festival where an author was giving away her book. I grabbed one with the idea of reading it but the reality was that without a bit of background on the book it was hard to spend the time to read it. It sat on my bookshelf for many years and eventually I gave it away.

It's true no one will read your book. So you better write it because it fills a need for you not because someone else will find it interesting.