Use Grammarly now, Grammarly tomorrow, Grammarly forever! Or at least until you suddenly find it unnecessary.
It's a very useful tool, a free app to be more exact, for those who are still improving their English skills but still want to write a book. It automatically appoints your mistakes in a text, even if it's a whole chap, and helps you correct them easily. It comes in the form of a keyboard on phones, and even suggests corrections while you are writing.
I made this post because I know there are many people out there seeking free proofreaders, which are difficult to find.
NOTE: Of course, like any other writing-correction app, it's not perfect, but definitely helps!