Forcing oneself to continue reading a book when enjoyment and interest has long since ceased still occurs today but I believe much less frequently than in the past. Except for students who are compelled by their teacher not only to slog on to the conclusion but also to answer questions as to motive and conflict, not to mention easily overlooked details such as similes and metaphors, many readers today allow themselves to close the book, never to open it again. Or in the case of e-books, to press the 'delete' button.
If you are still one of the few who sticks with the task, I wonder at your motive. You may have paid--perhaps quite a lot--for the book and are determined to extract your money's worth. Of course, you do not actually receive anything physical for persevering. It's not like eating a piece of cake that turns out to be dry and tasteless; in that case you at least become full and have extracted full caloric value. That you may become ill is a side issue.
Perhaps continuing to read means that you continue to 'kill' time, assuming that was your goal. Do people in today's busy world still need to eliminate time? Many of us barely have enough of it and others have addictive games on their cell phones, like Candy Crush, designed for that purpose.
You might be one of those people who is determined to complete every task they embark on and view it as a sign of weakness to do otherwise. I wonder if the gardener in the topiary above somehow couldn't quite figure out how to lay down the garden shears.
It might be someone you know and respect recommended the book and you feel you must give it every opportunity if only so that you can declare that you read it from cover to cover and still found it unsatisfactory. There's always the hope that the book may improve. Some best sellers are known for being slow starters that end up rewarding those who didn't abandon hope.
More common, I believe, than physically and vocally terminating the reading process by slamming the pages shut or loudly declaring our intent to stop, is simple benign neglect. Yes, we are still reading the book, we inform anyone who enquires. No, we haven't given up on it. We've just become busy with other matters. We may have even started reading a different book but we have full intention of returning to the first work. Of course, our friends and relatives forget about our plans or, if not, decide to be discreet and not enquire. If sufficient time passes we may persuade ourselves that we did indeed finish the book and perhaps our subconscious has even constructed a suitable ending, should anyone ask.
At this point, the most logical thing, of course, is to donate the offending item to our local library's book sale so that another person may turn its pages and, we privately admit, perhaps understand where the author was going much better than we ourselves did. If not . . . well, at least they got it for a reduced price.