I think Essential Grammar in Use is far too simple for you as a teacher of English. You could try Advanced Grammar in Use by Raymond Murphy.
You could also look at "Discover English" by Bolitho and Tomlinson. This is a novel approach to grammar for teachers. As the title suggests, you do exercises to discover the rules of the language.
You could also look at "How English Works" by Swan and Walters which is aimed at higher level students.
I hope this will be of help.