In the English language market of the book publishing world there are 4 million new titles published every year, about 75% of these are indie or self-published with the remaining million published by traditional publishers. The average American, counting ebooks, and paper, new books and second hand and loans from the library (which still pay the author a small fee).
The average American buys about and reads a total of about 8 books per year, for a total of about 2.7 BILLION books read (hint if you have seen the meme about how many Americans have read a book in the last year, how many since leaving high school, please note it entirely made up without any stats or sources)
Obviously, 2nd hand sales do not directly benefit the author, except a secondhand book for fifty cents to five dollars is a bargain way to meet an author you haven’t read yet and at $20 a pop for paperbacks, one might want to stretch their budget while testing a new writer. The other thing a secondhand book sale can do is it can free up both shelf space and partially reimburses the seller‘s book buying fund.
In any case, if one has a book that is both ranked 10,000th in sales, both in ebook and paperbook version, the author is going to earn somewhere over $50,000 per year. At first, the idea of 10,000 authors making over $50,000 per year at their craft, seems remarkable, but remember even if many writers publish 3, 4, 5 or more titles in a year, and 10,000, as a percentage of the 4,000,000 total tiles means 98% of authors are making less than $50,000, with hundreds of millions, like myself, not making enough to buy a bag of coffee to make at home per week, much less a cup at the drive through everyday. If you are getting into writing for the money, you will most likely be disappointed, it’s a bit like taking up a sport hoping to make it to the NFL/NBA/MLB and get rich. It happens, but you better love playing the game, because the real payoff for all of it is in the doing and being, not in the big cash checks you get.

Something to think about, when you buy a brand new 300-page paperback for $22.00, the author gets about $2 from the publisher, if the self-publish, they might gross about $5, but then they have to act as (or hire) their own editor, promoter, and publisher. So they probably make abut the same net income.
So, if you want to write a book, do it! Be sure to write the very best book you can, tell your tale, by the way, sometimes the best way to make a living at it is genre writing, I have many friends who write horror, scifi, fantasy, romance, especially non traditional romance, like MM, TM-TF, etc. they build a loyal following by delivering the kind of story a small but devoted group of people love and they write 2-3 books each year, and sell a few thousands of each title, and mostly the same few thousand people buy their new books, trade the old ones and follow them with great fandom.

But write your book, if you never sell a copy, you can spend a few dollars (about $200) at Lulu.com and get a hardback for yourself and your mother, print of a dozen trade paperbooks to give to your best friends and your 3rd grade teacher that either believed in you or conversely told you that you would never be a writer. You also can submit excerpts and short stories along the way. Or if you are like me, you can write poetry, the books are cheaper to print as they tend to be smaller, sales are mostly terrible, but again, if you are a writer for money, you probably ought to take up flyfishing as a hobby instead. But if you want to write full time and can figure out how to live off $50,000 or less, if you hustle, if you keep trying to write better and better, to learn how to market yourself, to learn how to build and maintain a fan base, you have a pretty good shot of getting there, but most successful writers I know and I know hundreds of them, didn’t quit their day job for a long time maybe their 3-4th book that sold well.
Me, in 60 years of writing, mostly poetry, I haven’t made a $1000, but that is because I love to write, not to sell.