You bought the book. You liked the book at first. Then for one reason or another, you abandoned it between 50 and 100 pages.
At least, that's the average spot according to Goodreads.com, which asked its users to tell them about why they put down books as well as identify the most-abandoned books.
(Sorry, J.K. Rowling, but apparently people don't enjoy “The Casual Vacancy” — even if they've purchased it.)