Sounds like a good place to share the "Twenty five books dropped by tryhards" infographic: https://s-media-cache-ak0.pinimg.com/564x/7d/80/4f/7d804f31c0039ac087ea5fe6d7966ace.jpg

I agree with Gary.

Any English grammar book should help. What aspect of your English are you looking to improve? English covers a wide spectrum of topics. Explore this website and ensure you know all grammar, vocabulary, etc. https://www.englishclub.com/

There is a book that I read in African American Vernacular English. There is so much slang that the book almost feels like a different language. It is called: "Their Eyes Were Watching God". The main character is a poor black woman from the state of Georgia so she talks incorrectly but when she narrates she uses proper English.

Why slang? That's a common mistake that English learners make. Slang is age dependant, country dependant, and unless you use it correctly, in the right context, you will appear strange.