I just read a fantastic post by Jeri Danksy of Jeri's Organizing and Decluttering News, titled Not Every Book is Worth Finishing. She quotes a number of writers and readers who give their cutoffs for when to put a book down. The main thing is to give yourself permission to stop a reading a book, and then give yourself permission to not finish it. I have a category I created just for these types of books on my Goodreads profile, since I do it so often. I love to read, and I just don't have time to finish books that aren't doing it for me.
Thanks, Jeri, for the wonderful reminder that not every book is worth finishing.