I wish Medium would pay me minimum wage for my time here. But it's nowhere near that xD

Anyway, I wouldn’t advise you to save quality content for a book — unless you are already very experienced and through lots of testing have figured out what works.

Instead, I would put everything out there. Keep in mind that you as the writer are often not the best judge of what other people will consider high quality. Also, I think it's a better approach to just put everything out, see what works with your audience, and then collect the winners into a book :)