If I am reading a book and if I am anywhere from 50 to 100 pages in and not feeling it I just do not finish it. I figure why bother. Unfortunately those books I do not remember what ones they are because I did not enjoy them.
I kind of treat a book like I do a new comic book series. I give it so many issues (usually 4 to 6) and if I am not like it then it gets dropped.