Fair Pay Denied
When you enjoy a book that you borrowed or bought used, the author receives $0.
Solution
You can tip the author for her work.
(Dislike tipping? Note that tip here is short for compensate an author fairly and directly).
5 Reasons to Tip
1. The Majority of Books We Read Are Borrowed or Bought Used
~68%1 of the traditionally-published paper books we read are ones we borrowed or bought used (including equivalents). The authors get paid nothing 2 for all that usage of their works. They should be paid something: median income from their books in 2022 was ~$10,000.

2. Used Book Sales Have Boomed
Because of Internet search and distribution technology, used books are now ~37%-45% of paper books sold and their sales are growing faster than new book sales.
"More than half of consumers within the US & UK buy more second-hand than new books." Used Book Sales Statistics, Dean Talbot, December 20, 2022, WORDSRATED
3. Authors Often Get Zero on Sales of Deeply Discounted, New Books
For authors, remainders, books being liquidated by publishers, and gray market books [NYTimes paywall] are the same as used. They're priced at ~50% off list — you see them at bookstores and online — and when you buy one, the author gets $0.
4. Borrowing a Book Is Great, But It's Also Free Riding
When you borrow a book, especially from a friend, you're using the author's work for free. Is that a fair deal? There's a simple way to make it fair: pay the author something!
5. When You Love a Book, You've Been Provided With Hours of Enjoyment
Whether a book is new, used or borrowed, if you loved reading it, consider paying the author a tip commensurate with the enjoyment or value you received.
ENDNOTES
1Borrowed + Used + Remainder Books = ~68% of Paper Books People Read
No one has done a publicly available, rigorous study of where people get their books. These calculations are thus back of the envelope. If you know of more reliable figures, contact us.
According to a Pew Research study, ~38% of the books that people read are borrowed, 24% from friends plus 14% from libraries.
So, of the paper books people read, ~62% are books that those readers bought.
Used books are 15% to 18% of the market by revenue.
Now, assume used books sell for 40% of the price of new books, on average.
That means, for each 1% of market revenue, used books sell 2.5 units per new book sold.
15% (of market revenue) x 2.5 = 37.5%.
18% (of market revenue) x 2.5 = 45%.
So, used books are ~37.5%-45% of paper books sold. For easy calculation, call it 40%.
As noted, ~62% of the paper books that people read are bought not borrowed.
So, used books are ~62% x 40% = ~25% of the paper books people read.
Subtotal: Borrowed + Used = ~63% of the paper books people read.
Then, remainders including gray market books are (pure guess) ~5% of the paper book market.
In Total: Borrowed + Used + Remainders = ~68% of the paper books people read.
2Authors Sometimes Receive Tiny Payments Per Library Loan
Public lending rights in some countries enable authors to be paid small amounts per book loan — around 10¢ in 2019 in the UK according to Wikipedia. Libraries support authors by buying books, of course, but the amounts authors effectively receive per loan are usually tiny.