I suspect it’s intentionally designed to be unpleasant to encourage book sales.
If you’re reading a book preview and have an awful time of it, why would that encourage you to buy the book? You’re more likely to close the site and move away.
The highlighted links are incredibly off-putting, like buying a second-hand book that someone has already highlighted. The readable text is so narrow and small, but the sections are so short that you immediately hit the massive "Buy a Book" upsell banner at the bottom, which distracts you. All of this makes it hard to focus on the text itself.
I'll never understand sites that have way too many links in their articles. Am I mean to click away as I'm reading a paragraph? Do you want me to stay on the article or do you want me to be distracted and go somewhere else?
Probably a mix of a style choice that didn't hit, and how pages were split up so it isn't as convenient as reading the book.
And the columns are painfully narrow which impedes readability. Overly wide body text is often a problem people recognize, but overly narrow is equally problematic.
It's a very strange experience to have to scroll to read all the text, even when there's barely any text to begin with.
If it works, they'd be dumb not to. (For the record, I don't think it works, and I don't think that was their intent.)