All books should be perfectly polished, well-edited, presentable in every way. But there are times when authors might be using incorrect grammar...on purpose. When is it not only okay to break the rules of language, but necessary?
When Bad Writing is Good
Sometimes, bad writing is needed in order to bring the setting to life. Ever read Gone With the Wind? Mammy's voice is clear and strong throughout the novel, and Margaret Mitchell does it with a lot of misspellings and incorrect grammar.
Many authors use incorrect grammar in dialogue and in stories with first-person narrators, and there's a really good reason for it: that's the way people talk. It's correct to say I don't know to where I'm going, but I sound like somebody who walked out of the 1900s when I say that. It's more natural to say I don't know where I'm going to, but this sentence breaks a classic grammar rule.
So when is it okay to break the rules, and when should you actively be trying to break them? When you're the way that people actually talk. Real people break grammar rules every single day, and if you don't believe me go look at Facebook for 40 seconds. I promise you'll find problems.
You should strive to write with correct grammar in beautiful English, of course you should. But it's even more important that you actually write the way people talk. Make your dialogue sound real. Make your narrator's voice feel real. Write the way people speak, and try to ignore the grammar rules you break along the way.
Save your beautiful, perfectly-crafted prose for third-person books, the Author's Note and anything else you're going to write in your own voice. When you're writing for characters, make sure you're writing for them. Write the way that character talks; hear them talking to you. Write that way, and your book will feel more real to your readers. It'll be even more beautiful than thousands of perfectly-edited and grammatically precise sentences.