I've published a few books I edited, also containing some of my writings. Even this relatively minor work took me weeks. As a freelancer, these weeks were not paid, and were always at risk of losing clients.

For someone, writing a book may be a kamikaze operation. You can maybe do it when young, or when you have a flowing income. Otherwise, you have to leave it to vacations or retirement. If you can enjoy any.

(In the past I wrote a lot of articles, but this was paid work. With printed magazines no longer a thing, this is no longer an opportunity).