I've scanned a book with my phone and uploaded to Z-Library before when it was a book I couldn't find online at all. Not a great quality pdf, but as someone who wanted to read the book for research and ended up having to buy a paper copy, I would've still preferred that pdf to nothing; it was still perfectly readable.

Like another commenter said, to get a good-quality pdf you'd have to take the spine off. Also note that it is really time-consuming and tedious to scan a book by hand (I assume there are machines that can automate it, but normal people don't have those). Big respect to people who do it regularly, but you may not want to be one of those people.