Pleasedeteme
J.K. Rowling probably isn’t going to read your book and critique it. You can read her books though. Pick any published book in a bookstore that interests you for that. Everyone has a different opinion, so it’s hard to give a name for the best author ever. For example, I tried to read Harry Potter several times and fell asleep each one. In my opinion, the books are boring and unreadable. However, the movies were decent. (I only say this because I know it is against popular opinion). If you want a list of top grossing authors in the world, I’m sure there is one online.
I’m not one of these famous authors, but I would say that good grammar makes your book stand out around here. Being able to understand your writing easily is essential to the story being enjoyable! If I have to guess what you meant because it isn’t written clearly, then it automatically is worse. Of course, you still need an interesting idea and plot that is well developed.
As for examples of what to read or do to improve, you can read and write anything to do that. When reading, look for what you like and dislike. Pay attention to grammar, style, imagery, etc so that you can take what you like and leave what you don’t. When writing, pay attention to feedback. Negative feedback is what will help you the most, so accept it and learn from it.
I can’t promise to proofread your work because, well, I’m lazy these days. There is a review swap thread though, if you want to post there. Some of the reviews can be helpful, others are useless flattery.
I can tell you that my style is to tell a story from one characters point of view. I think it makes the story the easiest to make immersive. For this perspective, you can pretend the reader is one of your characters and just explain what they see/hear/experience. This looks like, “You are walking down a dark, narrow hallway.” It’s also possible to take the perspective that you (the author) are a character who is relating what happened to you. This looks like, “I walked down a dark, narrow hallway.” The key is to keep it consistent. If the reader is a character, then they only can know things that character knows. Anytime the character does something like sleep, the story has to skip to when they wake up (unless it is a dream or they have a power to do things in their sleep).