There's nothing that will force me to turn away from a book quicker than poor grammar. I, myself, am not perfect. I want to make that clear. And if there is a typo, a mistake in my grammar, I would rather someone point this out to me. And I have been guilty of this numerous times. 

What really bothers me is when writers don't even bother to take the time to try and correct their mistakes. I have gone back months later in some of my books and have found mistakes and I quickly corrected them and apologized for my grammar mistakes.

I have noticed that some writer's struggles with...They're, their, there. You're, your. To, too, two. I did as well at some point. 

"Alex, you are going to the doctor.) (you're)

"Beth, this is all your fault."

"Amanda and Tammy decided they are going to the store." (They're)

"The apple feel over there, by the red ball."

"Carrie and Seth are so excited about their school dance."

"Mary's jeans slipped over her small waist.

"Throwing this away is just a waste of money."

"I love you too."

"I bought two candy bars."

"I'm going to the beach."

Just some examples to hopefully help us have a better understanding.