Link to April 3 Wall Street Journal post, "When to Put a Book Down".
What a coincidence. We were just talking about this topic in my SLIS class yesterday.
In my 20s and 30s, once I started a book -- always after careful consideration as I'm a big fan of reading lists -- I had to finish it. Now that I'm approaching 60, that rule has gone out the window. I've been known to toss aside a book before the end of page 1.