1) You can write it slow in your free time. No hurry.
2) You learn a lot and organize your thoughts in your head.
3) You can start selling before the book finished. It will allow you to test your audience and keep motivated.
4) When it's finished it's mostly passive income (though it's a good idea to also build a website and publish some blog posts regularly)
Here's why I think it worths it:
1) You can write it slow in your free time. No hurry. 2) You learn a lot and organize your thoughts in your head. 3) You can start selling before the book finished. It will allow you to test your audience and keep motivated. 4) When it's finished it's mostly passive income (though it's a good idea to also build a website and publish some blog posts regularly)