To Finish or Not to Finish...

Do you finish reading a book that is obviously not your kind of book? I usually do - it is though I have entered into some kind a weird contract with the book,
and then I think that I would feel as though I was letting down the book, the author and myself by not finishing it. That said, over the years, there have been a handful of books that I have not finished (and I am sure that neither the books nor the authors have held it against me, at least I hope not). I am fairly careful selecting the books I read, so I usually don't end up in the situation of wondering whether or not I should throw the book out through the window or leave it on a park bench for some unsuspecting passerby. But sometimes I can be disappointed, and then the question is: to finish or not to finish...