A book so emotionally draining you couldn’t complete it or had to set aside for a bit.
I haven't read one yet that was that emotionally draining, although, I'm eagerly awaiting my copy of The Fault With Our Stars, which looks like it's going to be a huge tear jerker.
I haven't had one that drastic yet. The closest I think would be Harry Potter, I could barely read the pages through the tears in my eyes. But, of course, stopping was definitely out of the question. For me, I guess, the more emotional is the more I want to keep reading. =)