There are very few books I have left unfinished, but there are books when I have seriously contemplated to just stop reading them. Usually it just runs into trying ro churn pages as fast as possible to get through it and look for something else ro read next. Few times getting past the first few hundred pages of boring text has paid of and the book has actually ended in my top list.
On the other hand when I'm reading a really good book at the same time as I can't put the book down I want to slow down for it not to end too soon. I need to hold myself back not to finish it too fast but often in those cases I find myself reading through the night, finishing the book just in time to have still a few hours to sleep. They can be quite captivating. Luckily this doesn't happen too often. There aren't so many books that good. I kinda wish there were, but then again that wouldn't be good for my sleep.