We would recommend to purchase a methodology book and a grammar book which you can refer to while doing the pre-course task, as well as during and after the course.
Methodology reference books:
Learning Teaching, John Scrivener (Macmillan)
How to Teach English, Jeremy Harmer (Longman)
Grammar reference books:
Practical English Usage, Michael Swan (Oxford University Press)
An A-Z of English Grammar & Usage, Leech, G., Conrad, S., Cruickshank, B., & Ivanic, R. (Longman)
Grammar reference sections of course books like: New Headway, Total English, etc.