how to make profit from writing a book?

I think I have a lot of valuable knowledge in a certain field and would like to pass on my knowledge to others by writing a book. (yes I know everybody thinks they have a great idea for a movie, app, or invention).

what's the best way of making profit from my writing? do I contact publishers to publish me a physical book? or do I just make a pdf and start selling it? what about people emailing each other my pdf for free, how do I protect myself?

If you want to make fast cash, make a pdf and
start selling online. That's what I did.
But even if you decide to sell a print copy,
I think the key thing is for you to get started
on the book. You can make decisions as you
go, based on the results that you obtain.

don't, you wont bro..

Look up automatic clients e-book

Sell your book on front-end facebook then make money with back-end