Since you are already a trainer, I am sure you have ample experience. Based on your experience and learnings from the industry—which are live and practical—you could go through the study material and attempt your assignments (which you must have received). In case you feel that the course material does not satisfy your thirst for knowledge, you could refer to a few other books. However, this would consume a lot of time.
Based on what I have learned and practiced in the last 10 years or so, as a student of ISTD, I would stick to the books and not go beyond them. Although, I would definitely refer to my own modules, etc., that I have from various organizations and companies I have worked with. This will enable me to gain insight into the topic.
Hope this clarifies your doubt.
Thanks and Regards,