I've never stopped reading a book because it was bad, but I've read two that I should have stopped reading because they were bad. One was Absolom, Absolom by William Faulkner - absolutely horrendous. He spends four pages describing the house in which one character will tell the other characters a story that doesn't happen in that house. The house itself is never mentioned again, but he spends four pages making sure we know exactly what it looks like. Why is this man revered?
The other one I can't remember the title of, but it was about a woman who receives her father's ashes, and decides to mix them into her red paint and start painting with it, and the things she paints start coming to life. Because it wasn't really her father's ashes (though she absolutely thought they were), it was residue from an alien spaceship that landed in the African desert. And spoiler alert, she uses this power to be a total dick. I actually thought about keeping this one on my shelf rather than turning it in to a secondhand store, because then there would have been a chance that someone else might actually read it. Seriously, I feel like reading this book made me a worse author, that's how bad it was. Also it doesn't actually end, it just runs out of pages.