Whatever the form, content by itself just doesn't pay a lot for most and never has. I just finished a new edition of a book of mine. It's valuable for "branding" (ugh term I know). But I'll directly make less money than from cranking out a blog post for some company.
There are a bunch of reputational benefits to certain types of book writing which (probably) lead to significant career/comp benefits. But the direct money can be less than what companies will pay for a ghostwritten content marketing-type blog. Or even whatever a lot of salaried people earn for doing that sort of work on company time. There's a reason a lot of "creatives" end up working for corporate product companies as their day job.