Not finishing a book (& in particular a good book) is only an option when you are re-reading it.

I have probably, by choice, not finished a handful of books (i.e. less than 5?) in my lifetime. Usually even if they are dreadful I have to finish them to make sure they have absolutely no redeeming qualities. I did this recently following a friend giving me a book that she hadn't read herself but had come highly recommended. I can only think those the recommended it were being paid to do so because I have read a couple of Mills & Boon (or Harlequin or whatever) that were better literature than that waste of paper. And then I was left with the dilemma about what to do with it. In the end I put it in the recycling bin where it might at least be made into some highly useful toilet paper.

The need to finish a book is so overwhelming that I currently have 2 in my desk drawer at work which I started reading my first year with my employer. 4+ years on they remain unfinished but of enough interest to me that I know I'll pick them up again. I will have to finish them soon given our company's current wish to close our department so I do have a deadline of sorts - then again the company did the same thing to my last department too so who knows - I might still have them in my desk drawer in another 4+ years.