A friend of mine told me that, and I've realized how true that is.
How many times have I gotten myself a book, settled down with it, and gotten half-way through before I realized, "This book sucks."
It used to be that I would force myself to continue with the book, and suffer through it, thinking, "It'll get better."
The only time, recently, that that happened to me was with The Clash of Kings. I was halfway through and thinking, "This is just so tiring for me. It's got to get better." In some places it did, so I read those parts.
So out the 50 or so books that I've read so far this year, only ONE gets better. I don't like those odds, so I decided this year that if a book sucks, I'm tossing it.
How do you tell if a book sucks?
1) If you're counting pages.
2) If you're arguing with the narrator.
3) If you realize the secondary characters are better than the main character and you skim the MC's point of view to get to the secondary character(s).
4) If there's such a complication that seems impossible for the MC or even secondary characters. (Simon Green almost...ALMOST did it to me, but I pressed on...and it got more impossible. But the writing style kept me going.)
5) When the book goes "Deus ex machina" on you. It means the author saves the protagonist...impossibly.
6) When you know, you just know, that the author is writing a Mary Sue.
Let me give you an example: Twice Burned, by Jeannie Frost.
Now, I loved the first book in her Dark Prince series. The first book I plowed through in two days, and stayed up late reading it. I was chomping at the bit for the second book, and got it a couple of months after it came out. I don't usually read the next book in the series right after (because I get burned out), but this time I did.
I was looking forward to the end of the chapters (1). I found I was doing (2) an awful lot. My Kindle highlights tons of parts that I was arguing with the narrator, calling her stupid, and insipid, and why-doesn't-she-accept-what-she's-got nasty words. I wanted to know more about Maximus than the idiot gymnast(!) who was narrating the story (3). Exactly at the half way mark, she had "died and gone into a coma" and Vlad's blood saved her (4/5). That's where I stopped reading. I have no idea if (6) came into play.
I'm sure you've seen this kind of thing happen in your own writing. Just remember to lead up to it. Please, for the love of Pete, don't kill your character all of a sudden and have her be reborn (unless, of course, she's going to be a vampire or zombie or something). If you're going to kill them, kill them dead, dead, dead.
Meanwhile, write your Mary Sue somewhere else. Like here. :)