I need help for improving my grammar.

English isn't my first language, everything that I know is what I learned in school. I'm currently studying to be a engineer but the thing is I like writing as well. I write blog posts for a website and I also write some poetry occasionally. The guy that I work for wants to write a book, and he wants me to help him. However, to be able to help him with that, I think I need to have a really strong command over the language. I would appreciate it if you guys can list the resources(sites/books) that I can use to work on my grammar.

I know reading helps a ton, I'm already working on that.